Cooperative bosses requiring multiple player groups. A group of bosses (3+) that work together to kill players. Each individual boss would have a different cap, so a different group would be needed to kill each.
Examples of what I mean by working together:
Boss A heals Boss B, Boss B heals Boss C, Boss C heals Boss A. With varying heal amounts per round or per a number of rounds (heal every 5, 10, or 20 rounds).
Boss A Heals Boss B to X% HP boss B every T time. Boss B does same to C, C same to A.
In addition to either of the above, possibly throw in:
Boss A Mana burns those attacking Boss B. B to C, C to A (as above)
Same, but with paralysis
Same, but with cooldown
There's other possibilities, heal and/status effect being the ones that appear to be the simplest implementable. Summons, harms, or even ranged attacks by the bosses in the circular pattern (A helps B who helps C who helps A) would be neat also, but as they aren't a current game mechanic would require more work from Radu, and even the suggested status effects would probably require some doing.
Possible candidates for group bossification (caps are very loose suggestions):
Small Gargoyle, Medium Gargoyle, Tall Gargoyle (60, 80, 100 cap respectively?)
Skeleton, Armed Skeleton, Phantom Warrior (80, 100, 120?)
Male Goblin, Armed Goblin, Hobgoblin (80, 120, uncap?)
Troll, Ogre, Cyclops, Frost Troll, Giant (80, 100, 120, 140, uncap?
There aren't any current bosses or special creatures that are reskins of the first three groups listed, so there wouldn't be any visual confusion of whether the new creature looks like a current boss or not, the last group clops/giant already have some specials associated with the skin, but... meh?
Technical hitches (for Radu, or someone else with EL code knowledge):
Can mob AI force the groups to remain near one another (otherwise one can just be lured away and then its easy).
Mobs have limited spell capabilities, can those abilities be extended to include some sort of heal? A remote heal or heal ally clone to work on monsters and cast by monsters, or some modification of the current smite summon?