Just a thought, how about adding some more mechanics for each of the boss fights, and have the mechanics change depending on difficulty? If not a flat gameplay change (minor new changes), maybe a new game mode that spawns you outside the encounter, lengthens boss fights and adds new mechanics/phases of fights? (in the future after the majority of current issues are fixed)
so for example
Recruit: fewest mechanics/complications, almost straight tank/spank
Veteran: 1-2max new attacks, adds, or mechanics
Champion: additional attack pattern/abilities, new movements/strategies for kills
Legend: everything from above + new dialogue/phases, more complicated attack patterns, higher skill requirement and teamwork efficiency.
I love boss fights and want them more exciting/heart pounding. These are the big badasses of the factions, i want to be scared