I think this is a really great idea. Ideally, I think fighting bosses would be like a puzzle. There are a variety of complex, interlocking elements, each randomly generated, and you get a new, emergent challenge each time. I think simple room modifiers are an elementary step in that direction, but I'd like to see things that required the more complex spell modifiers (e.g. upcoming ability to add effects, like hitting multiple target) add something, too.
Some other things to help draw in that direction:
- More complicated boss abilities (Auras that damage you if you get close. Maybe the amount stacks over time, so you can run in for quick attacks, but can't stay in there)
- Boss rooms that synergize with adds (e.g. platforms that sniper skelebots, if any, start camping on)
- Timed traps/environment effects that limit your travel through the room
- Bosses that switch tactics depending on adaptation/random chance at certain health checkpoints
-- Maybe you can search the keep/lieutenant keeps for intel on how that would change