I myself have wanted a sort of reintroduction of MP usage to Bard's gameplay, but have always been considering how it could work given what we have.

Few ideas i've tossed around is Repertoire procs being the thing which regenerates MP and having something to do with Soul Voice.
Maybe give us Foe Requiem and make it a personal DPS buff or smth that requires Repertoire to maintain?
Maybe make it a timer-based thing that replaces Soul Voice as a (Get 3 stacks of 20 second-proc things to use Apex/Blast Arrow. The amount of procs you get increases the potency?)

Kinda just tossing around a few ideas here!