What makes a great teamwork boss?

Capped attack speed for players. Cannot be surmounted by potions, speed infusion, boosters, etc.
Capped damage. At a very low value like 50k. Can only be broken by temporarily breaking through its special defenses. This is only done by triggering a special weapon that is part of the room (attack the reactors spread throughout the room)
Teleporting and other skills are disabled or greatly nerfed, however there are teleporters placed across the map.

To ensure that people are actually paying attention instead of just mindlessly attacking, implement that Pierre mechanic where randomly chosen players end up healing the boss if they attack while marked.


What else?

a boss that does 100% potential lock would be fun

Sounds like what they tried to do with Hekaton, where you had to move around and attack his parts in certain order to pass on.

Distributed damage attacks, like Bloody Queen's fire breath, might be nice, in terms of encouraging team work.

No rape-cage mechanics like in Hard Hilla

Aggro-options would be nice, like other MMO's where the boss focuses on only one player so everyone else can be safe while the Paladin/Bam/DA/DS/W/e tanks the hits.

It's not that there aren't any good ideas in the current game mechanics, but most of them are too extreme i.e. have a very small reaction window (if any at all) which is affected by lag or have no significant effect at all (poisoned? who cares).

(Re)implementing class diversity would already open up a lot of options.

For example:
Magic damage vs Physical damage (high defense versus one or the other)
Range vs Melee (range needed to hit a certain mob; boss causes high damage/OHKOs at short range)
High damage hit per line vs low (an "armor" stat or buff which reduces damage for each line of damage)
Mobility (fast teleporting mob/boss vs a slow one)
Burst damage vs slow, consistent damage (a la Vellum)
Survivability or utility vs DPS (imo survivability [e.g. defense, avoid, potion system] in Maplestory needs to be entirely reworked)
AOE vs single target ...
ETC...

If the dev team actually tried, I'm sure they could come up with even more ways of differentiating classes. From that point, it's a relatively simple matter of implementing things which immensely favor, but do not require, different classes. Arkarium snakes is one example (which was slightly flawed in execution). You could have other things, like a turret mob which can only be hit by long range attacks (but not AOE), and if it's not killed it buffs or heals the boss. Or, a boomer-like mob which forces targeting from players: it starts off fairly weak on its own, but if they aren't all killed together within a window, they keep multiplying to the point where you can't kill them or the boss.

As for class-agnostic methods, RA already has a bunch of good ideas. I also liked the concept of original VPQ bombs, where a player got a bomb attached to him/her and would explode, killing all nearby party members, if other members didn't stop to help.

I definitely feel that chaos root abyss bosses are a step in the right direction, as far as players having to pay attention:

-As mentioned with Pierre and the hats, but also when he splits into two and trying to keep them separated can be a bit of a challenge, especially if you're with a team. Add into that the fact that the Red Pierre is pretty glitchy when he disappears and then appears to 1HKO, ensuring that people need to be paying attention.

-I feel that there should be a boss that is only weak against holy (maybe I'm biased but I hate Ariel, dammit)

-I think there should be guild/alliance only bosses..or even an alliance system that has perks like the guild point system does.