Wyatt went nuts, like Joaquin Phoenix nuts. The Kal Penn idea is actually a good one, but the fact that he worked for President Kenyan would be too polarizing. If Bee is bailing (and she was not really good enough for that spot) then Jason Jones will go with her. Of the old crowd, I actually would not mind Rob Riggle in the spot, even though he leans more to the right. Even though she would never do it, Tina Fey would be perfect in that spot, too.
Ultimately, the best thing for them to do long term would be to start over with another unknown with a full new cast. There are some good edgy comedians out there who might be able to do what Stewart did. Chris Hardwick stands out as one possible guy, at least to me.