So first of all, I'm curious what types of crowd control are planned for the game? For example, stuns, holds, silence, slow, slow down cool downs, knock down(more or less stun), etc etc.
One thing I suggest for crowd control, particular hard CC like stuns and holds, is for there to be diminishing returns. For those who aren't familiar, it's where if the same type of crowd control is applied within a certain time frame, or if the same move is used, it becomes less effective in the duration each time it's used until a certain amount of time has gone by. This would only apply to player vs player however. I noticed a lot of mmo's make this mistake of not having diminishing returns for CC in pvp.
Another thing I'm curious about is how like effects will stack, like buffs, debuffs, and CC. For example, lets say there's a bubble defense ability, but 2 people have it or something like it, and lets say it gives the party so much of an armor boost. Would both bubbles stack, or would it it apply only the superior shield or perhaps give a diminishing stack based off of the lesser shield(s)? Then you have to think if the game has a raid like instance or something like what task forces were, where you have a group of like 20-25 people. How does one balance offensive and supportive effects, without making players completely useless with certain builds because they're build is slightly weaker than another players due to min maxing or just being a level lower? But also managing to prevent players from abusing stacking effects?
My suggestion, which the numbers can be case by case, is to have diminished stacking effects based on the number of similar effects. Lets say 4 people in the "raid" can apply a movement speed slow to mobs in an aoe, and it's 40% or something, obviously if this stacked, the mobs wouldn't be able to move at all, so I think that each player should have the layered effect diminish by a certain percent based on the number of stacked effects.
Not only this, but I think there should be hard caps like in old school coh on how much you can be slowed, etc.
Then you have things that have a duration, like a stun or silence. Lets say a certain mob enemy has the ability to lower a players armor by a certain amount, so taking care of that is crucial to survival. 3 players have the ability to silence that mob for 3 seconds, and that mob prioritizes on using that ability when it is able. Do you allow the abilities to add to the total duration regardless of how they timed it? Or make it to where they simply refresh the duration, so if they do it too early, it's their fault for not waiting to get the optimal stack? This one for me is a bit trickier.
I'd like to hear your guys thoughts on this, I felt city of heroes had a lot of inventive approaches to problems like this, a LOT of which that WoW actually straight up copied.