Yeah. I could totally see those two working together. After all, we know Mumford is still around since in a recent ep, Grover talked to him off screen. They could have a segment with Mumford, even if Jerry wasn't available and they had to find a new puppeteer.
Anyway, since I've seen more shows with Abby, I'll change my opinion a bit. It doesn't seem like she's overused too much. She does seem to appear in every episode, but it's mainly inserts unless she's in the street scene. She doesn't appear to be taking up all the screen time the way Elmo has. I think she's fine as 'just one of the gang.' The reason, I feel that Abby could be used less is because Big Bird needs more airtime this season. Some scenes with Abby could be replaced with scenes with Big Bird. I'd like to see them have equal airtime if you know what I mean.