Breaking the "Illusion of Choice" issue with MMOs isnt actually to hard, I would think.

The biggest issue with Bosses and Boss fights is theyre driven by DPS. More DPS, faster boss dies, less mechanics to deal with = easier fight. The way to break this is to relax the focus on DPS as the main progress point by introducing boss fights driven by mechanics.

As examples, we currently have fights of this:

"Kill Target" - Main goal of the fight is to dps down the target. All mechanics in the fight deal with making it harder to dps down.

But we could also have fights like:

"Survive" - Boss fights are intentionally hard hitting tank buster fights where the goal is to survive mechanics for an allotted time. DPS can be used to ease certain aspects, but doing maximum DPS will not end the fight 'faster'
"Protect" - Goal of the fight is to protect a Target or Targets from Damage through the course of teh fight. DPS can help mitigate out going damage against mobs (maybe), but would more be in service of other aspects in the fight, like independant DPS checks per DPS, or performing actions during teh fight to ensure the targets remain alive (holding certain positions, tanking certain adds even as a DPS, etc). You can use DPS threat itself to act as a threat mechanic, while discouraging Tank mobbing by putting debuffs on players tanking more than x-amount of adds.

All these fights could be mechanically heavy to force more emphasis on being able to do fight mechanics, and less emphasis on composition being the "100% ideal". While it wont eliminate the min/maxing crowd who want only the most ideal comp, it may help mitigate the need for "You got to have x, y, and z to do this fight".