I'm getting tired of individually responding each time I see these ideas show up, so I'm just going to post one huge list of all the item concepts that are imbalanced for whatever reasons apply. I'm not trying to kill off item suggestions, but nobody bothers to use the Search bar before posting their "original" item concepts, and then they usually take offense if I'm the first one to tell them their idea is neither original nor balanced. To those who understand and appreciate Riot's item balance (such as axesandspears), please feel free to cover anything I missed, correct my mistakes, et cetera. I will edit and give credit.
Also, please note that most of this list is based on Summoner's Rift. When I initially made this thread, I really had no clue that Riot was about to rebalance Twisted Treeline. Thank you, Rasmenar, for pointing that out.
1. Unnecessary mirror item concepts/items for the sake of items
-ArPen Boots (ty Nestrinast)
-AD Lich Bane/Zhonya's/Abyssal/Chalice
-Armor Wit's End/FoN
-MR Thornmail/Guardian Angel
This section is covering very specific suggestions. Often, people think "Well, why isn't there one of these for AD/AP champions? That doesn't seem balanced to me." Usually, they're wrong about that; items are balanced differently for AD and AP because AD and AP work in entirely different ways.
AD is usually associated with auto attacks, scales on 4 main offensive stats (AD, AS, ArPen and Crit), scales very well into late game, and (due to the fact that you spend more gold per point for AD than AP) on the few spells that scale AD, their ratios are always higher.
AP on the other hand is almost always attached to spell scaling, works with 3 main offensive stats (AP, CDR and MPen), and does not scale very well into late game despite being less expensive because very few spells scale above 80% of AP, and most of those that do are ultimates.
Thus, you can see why what might work for AD would either be overpowered or useless for AP, and vice versa. The stats work independently from each other even on hybrid champions.
Now, there are certain mirror items that are fully necessary; Rylai's and Mallet, Void and Whisper, Archangel's and Manamune, so on and so forth...however, these all exist because of very real itemization needs. Riot prefers to avoid mirrored itemization, but it is sometimes necessary and/or the best option available.
1.5. Broken concepts
-Magic damage return
-Anti-Crit (ty Nestrinast)
Some of the item suggestions in this section also fall under here; there are some item concepts, such as MR Thornmail, which are truly broken due to multiple factors, such as balance and coding. In the specific example of MR Thornmail, this item concept is broken
because AP does not operate in the same way as AD.
The main issues with MR Thornmail;
-AP tries to kill in bursts that have long CDs; MR Thornmail would increase your survivability so that you can survive their first rotation for sure, while dealing free damage back.
-Spell Vamp is far less available than Lifesteal, especially if you're an AD caster.
-AP cannot "undo" their spells; if they toss out a DoT or a full spell rotation on someone who has MR Thornmail, they have to just take the damage; they can't just stop after 100 damage and realize they should target someone else (and it gets even worse for skill shot champions, who can't choose their targets).
-AP does not scale as well as AD into late game. While most AP ratios are around 50-90%, AD is generally dealt in 100% on a very low cooldown (attack speed), and they can be modified to 200-260% very easily.
-What are you planning to do when the enemy carry buys Thornmail, MR Thornmail, and Infinity Edge?
-Mechanically, all non-auto attack damage is treated as spell damage. This means that damage reflection from Thornmail and MR Thornmail would also react with MR Thornmail's return. So, using 15% as a value for damage return (the most often suggested value), if two mages toss spells back and forth at each other with MR Thornmails equipped, each spell that deals more than 75 damage would deal its initial damage, and then 4 damage return ticks between the two mages--due to damage calculation going all the way down to 0.001--per hit. The amount of processing it would take for two mages' full burst rotation dealing that many ticks of damage would crash the game.
2. Imbalanced concepts outside of individual champion balance
-Missile Speed/Range increasing on auto attacks or spells
-Hard CC effects
-Increased CC/effect duration
-Invisibility (ty NineballCirno)
-True damage (ty ProfDrDeath)
These sort of effects are concepts that sound like a lot of fun, and I'm sure you can think of a champion who would synergize well with them without being overpowered. However, the problem with these effects rests mostly on the champions who have these effects innate to them, and then partially on champions who don't have them innate, but could abuse them so very harshly. Yeah, sure, Sivir would benefit greatly from a range increasing item...but so would Caitlyn. Depending on the stats, a Mana Vamp/Steal/Burn effect in an item could be either very overpowered or useless.
2.1 "What's wrong with Mana Burn?
Here's the long and short of the matter; Mana Burn is completely binary. Every time you try to depend on Mana Burn, you'll end up in one of two situations; either you'll perma-silence your opponent because they don't have enough Mana, or you won't be able to stop them from casting their spells and still being effective in the fight.
Another problem with it, pointed out by WonderBoy55, is that flat Mana Burn is more effective against non-mages than mages. This is due to the fact that carries and tanks are the two classes least likely to stack much Mana, while supports and mages will need lots of Mana.
Since flat Mana Burn is out, the next thought is "why not a low %?" Well, again, this is because it's binary; either the % burned per hit will not burn enough Mana to be worth it, or it will consume their Mana bar too quickly, preventing them from being able to cast spells for the team fight. There's a whole lot of discussion that can be made about Mana Burn, but basically, I never expect to see it showing up on an item; it might one day be used in a champion's ultimate spell, if anywhere.
2.2 "But what about Atma's Impaler? That's a stat conversion, and it's not imba."
It's true that Atma's Impaler is relatively balanced, but there are a handful of reasons for that. First of all, Atma's was designed way back when Bruisers almost didn't exist, and Carries would use it when they wanted Armor and damage, hence the Crit Chance on it. Second, when it became mainstream, that was due to the sudden influx of champions with AD-scaling ratios in their kits; due to how Atma's is designed, it's efficient on any champion regardless of their other items, so it became popular on everyone with AD ratios. Finally, Riot rebalanced it to where it belongs: a niche Armor/AD item that doesn't give you a BF Sword for buying Warmog's.
Personally, I would not be surprised if Riot, sooner or later, replaced Atma's Impaler, or changed it from a stat conversion. Stat conversions are much more balanced on individual champions.
The main problem with these suggestions is that they are binary balance factors. Certain effects that are balanced on specific champions become binary issues when available to every champion; HP>AP conversion is balanced on Vladimir because he's designed around it, but how powerful would it be if it were available to champions like Singed and Cho'gath. Range increasing effects like Bio-Arcane Barrage are balanced on their specific champions, but would be weak on one set of champions while strong on another.
The problem with these item concepts, then, is that there are only two ways these items can be balanced. Either they will be usable on all champions, but particularly overpowered on certain champs, or they will be useless to everyone and just barely usable, if not useless, to those certain champs. There's no middle ground; ergo, binary balance.
3. Imbalanced concepts due to stat availability
-Too much of one stat
-Too strong combination of stats
-Available to champions who would become imbalanced with this stat
-New building block tier items (ty Benjamander)
-Stat sticks for the sake of stat sticks, particularly MS bonuses (ty Nestrinast)
-Negative stats/passives (ty WonderBoy55)
The problem with these item concepts is that they play around with stats too much, trying to make the perfect combination (usually for one champion) to abuse at maximum strength.
There isn't going to be a stronger Armor/MR/HP item than Aegis of the Legion because having all defensive stats in a high value would be overpowered for tanks (hell, for EVERYONE. Carry drops a bit of damage for this in one slot, and they're undefeated AND deadly). There is also not going to be an MR item that combines 2 Negatron Cloaks because MR needs to be less available than Armor by design; magic damage falls off harder than physical damage in most cases, and almost all magic damage is dealt in bursts with long cooldowns.
The last one I mentioned, making a stat available to the wrong kind of champion, correlates mostly to CDR. Ranged AD Carries are specifically balanced around having 0% CDR in their item build (which is why Youmuu's, Zeke's and Soul Shroud suck, and Stinger and Brutalizer don't build into any stronger AD items). If they were to have CDR readily available to them, Riot would need to rebalance EVERY RADC because now they have to account for CDR that is readily available and does not eat into their build (because RADCs need efficient damage output).
4. More of an item stat that would become overpowered in high values
-GP10 (particularly boots, ty Nestrinast)
-New ArPen/MPen (ty Kagami)
Seriously, there's enough of these item types already. We do not need more of them, unless we want to nerf the existing ones, or make the items that share these stats have no synergy whatsoever other than that one stat.
Question about point #4, specifically about spell vamp. If spell vamp in large amounts is troublesome (which I don't doubt) why can't spell vamp from items be made globally unique, like tenacity or them MS from boots? Spell vamp from within a champion's kits, runes, and masteries would still stack with a single spell vamp item, as to not make it impossible for Morgana, Ahri, and Akali from getting spell vamp from outside their passives.
Would there be any bad side effects of this? I can't think of anyone who actually builds multiple spellvamp items right now anyway and it would allow for future addition of spellvamp items without breaking several champions.
Xypherous on Spell Vamp (ty ProfDrDeath)
Riot has specifically stated that they would only go as far as runes and masteries for Energy, and Fury is not going to be touched either. All manaless champions have to deal with "wasted" mana when they buy items; it's the price they have to pay for the advantage of not needing that mana.
6. Items/consumables that have effects on turrets
This is not a Tower Defense game, this is a Tower Defense game. Your tower might save your life sometimes, but the goal of the game is for you to protect your base, not the other way around. This is the reason that you cannot upgrade or rebuild turrets, or put up temporary replacements.
As for why there are no items with specific turret killing functions, this is because of two things.
First, the game is coded so that the damage you deal to structures is equal to your base AD plus the greater value between your bonus AD or 40% of your total AP. This makes it so that, even if AP champions don't build AS, they still have some scaling against turrets to help them push.
Second, the existence of Sheen is a veritable threat already. It is the only on-hit effect that is physical instead of magic, and one of the few that is allowed to proc on turrets. It only gets better with Trinity and Lich Bane...
there's a consumable item on Twisted Treeline that increases your damage to turrets. Your post is a little dated now.
7. Excess utility items
-Ward/Potion/Elixir pouch (ty Nestrinast)
-Support free ward active (ty Nestrinast)
Supports don't need a free ward item; they have GP10, and can usually build 3 of them early game and still be a solid threat.
Ward pouch is an unnecessary concept; it's your choice whether you want to keep map visibility or max out your build.
All the current potions/elixirs are balanced and complete their job, and the concepts for new consumables are usually for stats that would be very strong, such as MR/Armor or SV/LS.
8. Unnecessary upgrades paths
-Doran's upgrades (ty SnesController)
-More Sheen upgrades (ty ProfDrDeath)
-T3 boots (ty Pitufito Dell)
-T3 upgrades to non-Mercury Treads Tenacity items
-Upgrades for the sake of upgrades (ty WonderBoy55)
-Upgrade to legendary tier items just for the sake of 60+ minute games
-Item combinations that would render current combinations useless (i.e. Brutalizer+Zeal)
All the item suggestions that fall under this category usually pertain to upgrading an item that either already has good stats, or they have a stat that's powerful enough to warrant losing late-game strength for it. Or, in the case of Doran's items, something that's specifically designed for early game and only early game.
I thank you for taking your time to read this post, especially if you contribute to it.