What if we instead fought many enemies? Not trash. Instead the multiple enemies are the fight. None are trivial, teamwork is needed, but it's not just one big bad.
As players we're almost always on the losing side of Conservation of Ninjutsu. We're a giant pack of nobodies who think that because there are a lot of us, that we can do something. Nine times out of ten we fail horribly.
Let's have our enemies be the ones with numbers. They won't be total pushovers and they won't outnumber us ten to one as AoE fodder. But a player might take on 2-3 of them at once, and not have an easy fight. They'll have to balance their avoidance and damage; perhaps with an energy pool, mana, or just competing cooldowns. There won't be a lot of generic "don't stand in this" mechanics which are suitable for nobodies, but not heroes. We are supposed to be heroes, right?
Certainly some enemies should be stronger than us. But they should not be every boss. Instead what I see is the complete contradiction of what I said earlier: The revival of tanking. The fight has 25 players going up against 40 or so enemies. Some players won't be in direct combat, acting as healers and casters, behind the melee and trying to avoid contact. The average melee will then be taking on 2-3 enemies at a time.
Think of when you're soloing something that shouldn't be soloed. I assume you do this at least weekly, just to keep in shape. Do you just run in with all your damage cooldowns? Well maybe, but that's for trivial stuff. More likely you're saving combo points for a kidney shot, leaving some runic power in reserve for a mind freeze, that sort of thing. You're playing more of your class when you tank for yourself.
On the subject of CC and interrupts, why are bosses immune to almost all of them except in very specific cases like KT? Simply put, they would be completely overpowered. Wave of DOOM KILL is supposed to hit the raid, not be interrupted at a trivial cost. Can you imagine the usefulness of say, stunning a boss who has enraged and you just need a few more seconds? When there is only one enemy, CCs stop 100% of damage (excluding all the exceptions), giving a lot of time for tank recovery as well as a bit more DPS.
With more enemies, CC could be used in a balanced manner. Interrupts would be a way to moderate incoming damage, but not trivialize it, since Wave of DOOM KILL would instead be multiple enemies casting Wave of OUCHY THAT HURTS. We could use a larger portion of our classes in PvE. Currently we use only a small set of abilities in PvE, while in PvP we use many more. This is not inevitable, except when we are stuck with only one boss who must be immune to all the negative effects which make PvP unfun for the victim.