There is some really awesome stuff in this thread and I'll be checking in with you guys more later as well. I'll be working on some new items in the near future so it is really awesome to see so many well thought out suggestions. I don't like to comment too much on specific items but I've read through all the submissions and want to comment on some of the patterns I've seen.
1) - Counters to various team/game metas (Anti AoE, Anti Heal, Anti Burst, Anti Tank).
I am a pretty big fan of these in general. I think having stronger counter items overall encourages diversity in team composition. The trick here is making counter items effective against either one player who is dominating a match, or a full gimic team (team heal, or team beefy deepz for example) while not simply being just amazing items in general and failing their primary purpose- A perfect example of this is deathfire grasp. DFG is a great item but it looks like it should be an anti-tank item, However, due to the fact that tanks traditionally have very high MR values it is best used as an extra free nuke to blow up squishy characters even faster than before.
2) - More AP combinations (AP/Armor, AP/CDR)
This was addressed slightly with the Z-Ring split and we have another caster item in the works, though I can't say much on the specific details yet. I think that itemization was a limiting factor in caster end-game success although the Deathcap and Hourglass
have helped out this to an extent it is something we'll keep our eyes on.
3) - Unusual hybrid stat optimizations that don't tend to normally work together but may work wonders for specific characters.
These items are a bit scary for me to try and implement because they tend to be binary in nature. These items tend to become amazing, must have items on several characters but complete garbage on everyone else. This tends to not add much to item build diversity, which is something I'd like to improve in our game.
4. More upgrade options for "Dead End Items" (haunting guise, executioners calling, etc)
I agree that dead-end items can feel kind of bad for some players. Other players enjoy getting several mid tier items instead of rushing that big legendary item, I personally belong to the first group but we do need viable options for both types of players to have the item shop feel really great.
One issue that wasn't mentioned often in a lot of the suggestions was adjusting cost of some of these dead-end items if they were given upgrade paths. The cost of items is adjusted based on a large number of factors, one of those is if those items turn into something else more useful. Items like Haunting Guise would have to be reevaluated and would probably suffer a cost increase or a stat hit if they had an upgrade path. This would make people who like the current item feel pretty terrible. Adding more upgrading paths is something we're considering for several items like this though. I'm looking at Executioner's calling in particular since it feels incredibly lackluster. I can't remember the last time I have seen one used.
A good example of cost-efficiency is the Doran's items. They are fantastic values and the reason you get so much for their cost is because they take up valued inventory real-estate. If they did have an upgrade path we'd have to increase the cost accordingly which would weaken our item diversity. Doran's items I feel add a unique element to the game in terms of how one builds their character. Does one rush for the early game advantage or do they plan far in advance for the end game scenario?
This was a bit of a ramble but at the end of the day I agree with many of you, I would like to see more valuable dead end items (haunting guise) while having more cool upgrade paths. This can be solved by adding a few more upgrade paths while also revisiting many of the "dead-end" items and ensure that their purchase actually feels good and is a worthwhile investment.
5) - More "Pure" items - Similar to Thornmail in purpose but based around different stats.
Lots of good item suggestions here involving stuff like this. I've seen a lot of suggestions for an MR equivalent. I'll run some of these buy folks and see how they stick. I have no objection in principle to more "pure" items as long as they are priced and balanced appropriately.
6) - CDR, CDR, CDR
This is a stat that everybody wants and I feel we throw around too freely. I would like to see CDR placed on more items in smaller amount personally. With a cap of 40% it feels a bit odd to me that one item can exceed half the cap. Too many CDR items makes it too easy to hit the cap and severely limits your item choices (as well as making skill balance difficult). Once you hit 40% or close to it you don't want to spend more money on anything with CDR since it's wasted gold. I have no objection to offering more CDR items but I would like them in much smaller amounts than much of what was suggested here. Randuin's Omen with 8% to me still feels like a good item. I think more items with smaller amounts of CDR (<15%) could do really great things for our itemization.
7) - More team supporty items.
Auras were mentioned on a number of occasions. The problem with auras is that they are often incredibly powerful but don't feel like they are impacting the game. Items like Soul Shroud, Aegis of the Legion, and Will of the Ancients are absolutely amazing items but are very rarely used because they don't feel impactful. Items like Shurelia's Revelry though feel super awesome when used well. I would like to see more support items that feel really good to use/own in our game. That said, new aura items are certainly possible and could be cool, they are just something to be wary of when designing.
8) More active items -
I won't comment too much on these although I think there are some really cool opportunities here. We don't want to go overboard on these since the majority of our
players (even the top players) forget to use their actives constantly. Though I will say a few more awesome active abilities would be nice.
9) Anti-CC items -
Completely understand where this is coming from (in short - stunlock sucks). Xyph has commented on this a bit already but we had a new CC reduction system for items we'll be rolling out soon which should be really awesome.
Obviously these are my just my opinions and don't always reflect the design team as a whole. Hopefully though, this post was helpful. Keep up the good work.