It just so happens that I'm currently playing Tyranny, which has what is basically a bard join your party. Her music works by building over time: you start with a slight buff, but the longer you sing, the stronger it gets and at regular intervals you get a resource (I think they were called beats? Breaths?) that you can spend for a bigger effect, but that also starts the buff over from 0. It's quite a nice system.

I.e. Inspire courage: +1 bonus for everyone in the party, an additional +1 every round, to a maximum of +3 (more later levels), dismiss to grant someone [benefit dependent on strike used].