Songbird/Diamondback, there's a Thunderbolts thread and Songbird's own appreciation thread to discuss that.
Anyway, I'd probably mix it up with:
- Invisible Woman
- Ms. Marvel
- Machine Man
Mostly female characters, but the two dudes there that are bound to have entertaining interactions (if Aaron Stack isn't yout thing, that slot could be a revolving door). The composition means things can be fun but it won't be discounted as a fluff book (all levity, no purpose or lasting change).