Michael Scott is the focal character of the show. While there are other strong characters like Jim, Pam, and Dwight, I don't think the show can shift it's focus to one of them and hopes for it to carry on successfully. Dwight would be the best choice but that would only work for possibly one season.
If the show is to continue it's going to need some heavyweight comedy actor to step up and fill the void. Even though it would never happen, someone like Jim Carrey would be a good fit on this show.
Personally I think they should just wrap the show up with Carrell's last episode.