Personally I think next season should be the last.
However, IF they do another season after, and I'm assuming at best it could survive only one season without Carrell, here's the idea I thought of that might work:
Instead of making someone Michael's replacement, have several guest stars try to fill the role. For example, say they promote Packard to manager and let him do it for 1-2 episodes and then they decide he's not working. After that bring in guest stars that would last for about 2 episodes, like Stephen Colbert, Paul Rudd, you know those type actors. By the end of the season, they still don't have the replacement and have the series finale be Ricky Gervais coming in reprising his David Brent character to interview for the job and the same time Michael shows back up to say he's taking the job back.