Crowd Control:
I want crowd control back. Interrupts, slows, stuns, etc. Tie most crowd control to oGCDs, not cross role nonsense. Give certain roles/jobs better versions. For instance, Tanks likely have the best stuns. Best being defined as a measure of cooldown/cost (if any), duration, range, AOE vs. ST, etc. Melee probably get best interrupts, ranged get best snares/roots/slows, etc.
Interrupts:
Have bosses that cast things that can be interrupted. Tie these mechanics to less scripted timers forcing people to adapt and be aware and also have failure to interrupt not be instant death. I.e. damage spikes on a tank, they react by shifting to more defense and healers focus on more triage to stabilize. Maybe missing an interrupt spawns an add, creates a dangerous void zone, destroys part of room, causes enraged status for a brief period, spikes burst damage, drains resources, heals adds/boss (less strict hard enrage timers, but much more soft enrage design), etc.
Slows:
Enemies that need to be slowed to better control flow of incoming damage. Maybe allow bosses to be slowed and let a tank kite to give healers time to catch up/drop stacks, etc.
Stuns:
I want to bring in this concept of "CC armor" here. Have bosses do special attacks, again on not strictly coded timers, that require coordinated CC from the team to delay/prevent. B&S/Wildstar used this system to great effect IMO. Enemies and bosses would have varying levels of CC armor for their attacks and each job/role would have varying degrees of effectiveness in those stuns. CC would only be available during the boss' attack. Think of it like this:
Boss has 5 different basic abilities:
Tankbuster
Raid AOE pulse
Persistent Voidzone
Self Buff that grants damage buff and vuln down buff.
Resource Drain
Lets say every 45s or so the boss cycles through these 5 100% of the time, but possibly up to like 6 or 7 times.
The idea would be as a team you'd decide which of these are the most dangerous to you at any given time. Imagine your tank is out of cooldowns, and is at 26% HP. You see a tankbuster casting. By time it finishes the healer might be able to get tank up to 55% HP. It might be safer to CC that, ignoring the tankbuster, and eat the persistent voidzone that makes the room smaller/makes follow up mechanics harder.
What happens is the encounter becomes dynamic. You now have a smaller room, but safer health pools. What happens if you do this strat 5 times? Now you have very little room. That's dangerous. Imagine if over the course of the fight, you're close to killing, but the boss has gained 6 stacks of damage up. That means any raid wide damage or tankbuster going off could be a wipe. You prioritize those and push the DPS and try and clear. Failing a CC isn't a wipe. It's a choice that impacts how to approach the fight and lets you work around your comps. Imagine one raid night you're stuck with 4 melee. That means raid wide damage isn't hard to heal (everyones stacked), but space is paramount, so you focus on interrupting those abilities and eat the damage taken ones.
Tanking Paradigm:
I will go into more detail in the Paladin bit further below, but for now I want to touch more on the concept. Tanks have 4 main responsibilities. That is Threat, DPS, Mitigation, and Positioning. Currently I feel that NONE of these responsibilities are captured well. Threat is irrelevant, DPS is passive, Mitigation is one and done, and bosses RARELY move. Under my proposed paradigm A tank has to spend its resources on which of categories it needs at any time and that will shift multiple times. The other bit of design I'm interested in capturing is that a tank can adapt to his group. If his healers are weak and his DPS are strong, he can focus on covering for the healers. Alternatively, if DPS is bad, threat is less important, but damage checks might need help getting through allowing a tank to shift his resources to DPS expenditures.
Tank Swaps:
Ideally I'd like to see tank swaps be less taunt at X stacks or taunt to invuln Y. I honestly don't think that's good design. Better design would be a system that allows swaps to be more organic/dynamic as well as more defined roles.
Example mechanics:
Boss hits hard and frequently. You would alternate based on the inability for a healer to stabilize a tank's health pool, this would give time to let the other tank recover slowly over time.
Boss hits a tank with a debuff that enables them to do XYZ mechanic. I.e. Clear void zones, kill certain adds, survive XYZ. Maybe the debuff causes a rapid HP drain on the tank, thus having them tank and soak the debuff simultaneously, would be incredibly difficult to deal with.
Periodic add spawns during boss fight (not intermission style that is commonly used) that give a reason to swap tanks or pull a tank off normal duty.