Sorry for the double post, but it feels like the two subjects are different enough that they shouldn't be fused in the previous post.
I feel like the problem with just focusing on items is that it doesn't address the lack of identity the Support is suffering from right, problem that also make it one of the less desired role in the game. ADC get to revel in the damage they do, Tanks on the fact that they can't be killed, but Support as it is is only defined on its ability to starve well and ward.
Giving them some items with a bunch of actives and bad stats would not give them a defining feature. It would be another case of 'I can have the worst (instead of starving) and still be viable'. Also if someone wanted the active badly enough, they could take it and suddenly become a support too.
I do believe Supports should care about stats like everyone else, and everyone else should have access to interesting actives. I would be ok if Animal Transfiguration Device was picked by an AP because the active is what he needs right now. Supports should just be defined by there ability to sustain and actively enhancing their team.
Anyway, I agree it would need some work to make AP scale with support CCs and it could get ugly fast if the ratios are wrong (even though it could be mitigated by making sure Support don't have much damaging capabilities). Yet I do feel like reworking the couple (like maybe 12-15) support abilities that have a controlling component wouldn't be that much of a task. They would also just have to make sure the AP ratios for the damaging parts of the abilities scale really really badly.
I also agree that more items is always fun , however I think a higher priority right now is to give Support an identity and something to be proud of other than warding and zoning (which can be done by anyone else).
--- On Topic ---
Personally I would prefer Synergy being a Mastery in the middle of the Utility tree. As a mastery, it does a good job setting up what you plan to be/do on the battlefield, and it also differentiate Utility as the tree that define a support, someone who can profit from other succeeding, who has multiple tools at its disposal to affect the game in subtle ways (aka not through direct damage), that value movement speed, gold, CDR and overall utility, and someone who doesn't need or can't farm.
It would also give a reason for Tanks, AD or AP to get in the utility tree.
It's a two-pronged argument. One was that implementing synergy to give the 5th champion on your team (I'm intentionally not using the word support here) a source of gold income is a good idea. The other is that in order to maintain that that 5th champion can still be a support when they have income, we will need more items for supports. I was making a point that they implementation of synergy should come with a revamped store (hence why it was still on topic).
As to the actual implementations of the mastery itself:
Global Farm: Not a fan of this idea for a few reasons. The first is that is removes some of the sense of reward a player receives for getting a last hit. It's something that you accomplish personally, and it feels awkward handing the reward to the rest of the team. If I'm farming well, I still might not have a lot of money because mine is being diffused while my mid that is struggling isn't providing his fair share back. Why should I be punished because another player is doing poorly? It'd be awful for a player who is beating their lane opponent to still not be able to fight him or make plays because the rest of the map isn't on track.
This one technically shouldn't matter, but it gives more power to trolls/afk players. It's already hard enough to stay in a 4v5 or play with someone who is trolling. Now some of my money goes to them despite the fact that they are making no effort to play with the team at all? No thanks.
Synergy Item: Also not a fan of this option. If gp/10 is weak because it's backloaded, why would we introduce another item that initially decreases your gold value in exchange for more lategame gold? You aren't adding a different mechanic, merely another version of something that's already there.
Plus, if the point of the item is to give supports more items to buy, then I have to suck up yet another item slot with an item not devoted to actually making me poweful. Between this, boots, wards, and gp/10 still being a good idea since they do pay off in the long run and build into support items, where am I going to have room to get the actual items I'm saving money for?
Finally, while synergy is a cool mechanic, you don't want it to be on every member of your team. Even with diminishing returns, it's still going to be most efficient to run around the map as 5 the entire game, clearing creep waves as a team.
Alternate Mastery: Let the hate-train continue. The problem I have with this one is that you are effectively removing a mechanic that is a part of the game. There are still cases when supports take some CS. Sometimes, you have to cover a lane your jungle can't get to. Sometimes, your AD carry dies or has to leave lane, and you'll get some CS (at your turret). Sometimes, you go an clear jungle creeps that your jungler doesn't have time to get to. Sometimes, you want an early item so you ask your carry for a couple of creeps. Getting CS still matters on supports, and removing the option for them to take a little CS here or there not only isn't fun, but it's relevant.
Synergy Runes: I dislike them for pretty much the same reason you do. Runes are supposed to be a choice to allow you to customize some of your stats. They aren't supposed to be mandatory options to keep up in a game. The current gold quints are already a bit of a problem in this respect.
Basic Synergy Mechanic: I feel like Darth Sidius right now. This is the part where I give a more elaborate explanation for why I don't think it's a good idea for every member of your team to have synergy.
The problem is that is changes how your should be farming lane phase, and promotes/unbalances a lot of mechanics. Basically, you get more efficient gold/creep killed the more people you have in a lane. The example I used earlier of your entire team walking around together is a bit drastic (as it's difficult to move around the map that quickly), but I can give you a much more realistic scenario that is still problematic. So, your team starts out at blue, and your mid and jungle clear it quickly. Next, they both move mid and push it to the turret, with the jungle picking up extra gold for it. While the lane is pushed, your mid rotates top, and then begins stands by while your top pushes and takes the free farm for that. They both then move back mid, which is beginning to push back to their turret since they shoved it earlier. Mid shoves again while letting top suck up free extra gold. They then rotate back top and rinse/repeat.
See the problem here? You are over-incentivizing passive gameplay, because rotating and pushing just gives your team way more gold without any interaction from the opponent at all. Early gameplay simply becomes about coordinating pushes to get as many champs as possible in one place to soak gold at a given point in time. Why would you hang around your lane when you could make a run mid and get free gold? On top of this, in order to carry out this strategy, you need mobility and pushing potential. This just further increases the power level of these base mechanics on champions kits, essentially nerfing any champ that struggles to push or can't make it to other lanes.
Synergy is a good idea for supports, but it's not something you want to see around your team.
Basic Synergy Mastery: That leaves just one more option. I feel like this is the best of them all if you put the mastery very deep in the tree. Technically, it suffers from the same problem that every member of your team could grab it, but in this case, you are giving up other significant bonuses to do so, which makes it more of a choice. It could arguably still just be too strong of a mechanic, and everyone would still just take it, but it's closer.
If I were to implement it myself, I would put it fairly deep in the utility tree, in addition to making a modification to your diminishing returns idea from the original thread.
In your example, the gold gained only diminished if there were multiple allies in range. Your example from the thread:
Originally Posted by CupcakeTrap
Synergy: gain 2/4/7/10 gold when a nearby (1000 range) allied Champion kills a minion.
If another champion with Synergy is within range, you gain only 1/3/6/8 gold.
If 2 other champions with Synergy are within range, you gain only 1/2/3/4 gold.
If 3 other champions with Synergy are within range, you gain only 1/1/2/3 gold.
I would make a modification to make the range global on the diminishing returns. For example:
Synergy: gain 2/4/7/10 gold when a nearby (1000 range) allied Champion kills a minion.
If another champion on your team has gained gold from Synergy in the last ~8 seconds, you only gain 1/3/5/7 gold.
If 2 other champions on your team have gained gold from Synergy in the last ~8 seconds, you only gain 1/2/3/4 gold.
If 3 other champions on your team have gained gold from Synergy in the last ~8 seconds, you only gain 1/1/2/3 gold.
In this scenario, it keeps your team from just temporary grouping for free gold, since you would be taking gold away elsewhere on the map. This would diminish the effect enough to keep other teammates from sacrificing masteries to gain it, since the effect would be minimal the more champions that had it on your team. It's a bit wonky, but it's the best solution I've come up with so far.
That being said, it still has a potential problem. While it only really makes it beneficial for 1 (maybe 2) member of your team to have Synergy, it doesn't define which ones. There is no reason your mid and top wouldn't be the two taking this and then just push rotating to increase their gold gains, leaving your support botlane without the mastery active, since it's better for the top and mid to have the extra gold (better scaling).