In my opinion, they need to find someone from the outside. Everyone already has their designated characters that all bring a little something that makes the show what it is. I think that taking one of the existing characters to replace Steve Carell would be a big failure. For example, when Jim was temporarily Micheal's co-boss in season 6, he lost most of his character. Although there were some funny moments between Jim and Dwight during that period, there were less pranks which is part of why the show was so funny.
This is just a suggestion. But I think that Danny Devito would be a more than adequate replacement. If anyone has ever seen the show It's Always Sunny In Philadelphia, you might see where I'm coming from.