No one knows for sure what NBC plans to do to replace Carell, and while it's the preference of most of us to go ahead and shitcan the show while there's still an ounce of goodwill remaining, that's not likely at all, unless ratings next season go into the tank. However, there are a few interesting possibilities. For instance, should "Community" not make it another season, Joel McHale would be a fun replacement. Obviously, Jane Lynch would be the ideal replacement, although that's not likely to happen at all. I also like the possibility of merging the worlds of "The Office" and "30 Rock" and making Dunder Mifflin an entity of whatever cable company it is that owns NBC on "30 Rock" now, which would allow Alec Baldwin to, at least, fill a role similar to Kathy Bates' role this season. In that case, it'd be better, I think, to promote from within: Give John Krasinski the boss' job, although if I'm not mistaken, his contract is ending soon, too. I wouldn't mind seeing John McGinley take the role, as long as he spent most of his screen time delivering tirades against Dwight Schrute and Hugh Jackman. There's also the intriguing possibility of Ricky Gervais reprising his role, but American audiences would never buy into a character that unlikable. And if NBC were to go the safer route, they'd just bring back Melora Hardin (Jan) or Andy Buckley (David Wallace), who would make the most sense in that universe, but likely wouldn't provide the instant ratings bump that networks like about stunt casting.
Not that it matters, because none of the above possibilities are realistic. That's not the way a network operates: It doesn't go out and find new or unestablished comedic talent to replace the lead character in a flagship sitcom; networks find veteran, established talent. Someone with considerable face recognition, but who probably doesn't suit the show well at all. Even better if it's an actor or actress who has experience with slowly dying sitcoms. So, what I'm positing below are not the seven ideal candidates to replace Steve Carell, but the likely candidates to replace Steve Carell, given NBC's history of boneheaded decisions.
However, I will note that, while the choices below aren't ideal, because Greg Daniels will obviously have considerable say in the choice of replacement, he won't let it get to the Charlie Sheen/Jim Belushi/Christian Slater levels of badness. My guess is that they'll be safe, nostalgic choices that will draw, at least temporarily, an even larger audience than "The Office" is accustomed to out of simple curiosity before ratings begin to deteriorate rapidly.
Ted Danson: Danson's not actually the worst possibility, even if he doesn't exactly fit the tone of the show. "Damages" has proven that he can play asshole well, and "Bored to Death" has demonstrated that he still has some comedic ability. I think NBC would drool over the idea of bringing back someone from the glory days of Must See Thursday night, and there's a potential fish-out-of-water storyline possibility in Danson's character being a mogul of some sort who buys up Dunder Mifflin only to have the rest of his corporate interests fail spectacularly, leaving him with no choice but to run the paper company.
Jerry Seinfeld: Seinfeld is another former Must See TV possibility, and given the continued strong relationship between the network and Seinfeld, I could see this happening. Clearly, Seinfeld is interested in working again (hence, "The Marriage Ref") and the best way to kill the Seinfeld Sitcom curse is to attach yourself to an already established show (and run it into the ground, possibly). Again, tonally, it's not that great a fit, but this is NBC we're talking about. The same network that brought back "Knight Rider."
Will Arnett: Arnett is a very definite possibility, I would imagine, assuming that his Fox sitcom gets shitcanned by mid-season, the likelihood of which is high. I think his character would suit half the Michael Scott role -- that of a obnoxious doofus who lacks self-awareness -- but I'm not sure Arnett could be as obnoxious as Michael Scott yet still remain somewhat likable or display a modicum of emotional depth (not that Michael's done much of that in recent seasons, either). I think he'd be a great addition to the show ... for about five episodes, and then his Gob personality would wear incredibly thin. He's also got the close Greg Daniels connection since his wife is the lead in Daniels' other workplace sitcom, "Parks and Recreation."
Matthew Perry: Perry, like Arnett, already has a show in development, "Mr. Sunshine" over at CBS, and it's one with very good talent behind it ("The West Wing's" Thomas Schlamme is a producer, and Alison Janney is a co-star), but assuming that "Sunshine" fails to find an audience, Perry wouldn't be the worst choice, though our nation's eroding love affair with irony and sarcasm might hurt Perry's chances. But, then again, he's likable, has good comic timing, and he's instantly recognizable. I could definitely see NBC courting Perry for this role, but I don't know if Perry would ultimately accept.
Eric McCormick or Sean Hayes: If they bring in either McCormick or Hayes, they'll probably bring in a laugh track, too, otherwise, how would we know when to laugh? Actually, McCormick was pretty good in the short-lived "Trust Me," but he's likely too straight for the role. Sean Hayes, however, wouldn't have that problem -- his problem would be in finding the nuance of his character. But if he did play the character gay, wouldn't it be fun if Oscar and the new boss were the next Jim and Pam? OK. Not so much. But I'd bet dollars to really big, delicious donuts that were worth at least $.90 that one of these two names has come up with the NBC brass and Daniels.
Too bad "Parks and Recreation" has already pilfered Rob Lowe.
John Oliver: We already know that "The Office" likes "The Daily Show" alums, as both Carell and Ed Helms got their start on "TDS." The only anchor left on "TDS" that might properly fit the bill is probably Oliver, who did a fairly nice version of a dimwitted person of authority over on "Community." (John Hodgman is also an interesting possibility, but would likely lose his appeal after a few episodes, while the rest of past and future "TDS" anchors seems too lightweight for the role, save for Colbert, who's already got a steady paycheck). There's also the possibility of a lot of bad, broad jokes between Dwight Schrute's character and an English boss, something which I'm sure NBC would love.