I have a hard time thinking of another champion I want that wouldn't just be (more or less) filled by a rework of another champion (I'm really looking forward to the (eventual) reworks/relaunches of Poppy/Yorick, for example), or just an existing champion getting buffed or tweaked slightly, like Olaf or Trundle.
If Bard is a sign of things to come, I think the new champions are taking up a pretty unique space in the game going forward, where most established niches are just waiting to be filled out with existing champion's reworks or tweaks.