Never played .Hack, so can't make a comment on that, but this idea was inspired by FFX2, Lightning Returns, Devil May Cry. Maybe not IN BATTLE cuz that'll make DPS classes EXTREMELY complicated to play. But more like, big pull, ok switch to AOE savvy DPS. Boss, switch to Single Target savvy DPS. Demon Wall, switch to warrior, killed demon wall? Back to pally kind of thing. And if you can't do it, who the hell cares. Play 1 class for the whole dungeon, play it anyway you like. The idea is hey, I feel like playing this way for this mob, bam switched.

Keep the contents fun and different.