Then there is the chance that Ricky Gervais, who has become more well known in the USA since the Golden Globes, will take his spot which could make the show just as popular. I doubt he will though, it will probably be someone entirely new or existing like Ed Helms. Will Ferrell guesting in 4 episodes will be fun, but I doubt he would become a regular.
Of course it will be sad to see Michael Scott leave (I'm sure he'll be back for guest roles at some point though-Michael/Holly wedding has to happen), but The Office has the best chance of any show to continue, at least for a few more seasons due to a great, varied cast. Here's some "Prison Mike" for old times sake: