In this Huffington Post story on the cancellation of the George Lopez show, the writer gives takes on 16 comedians who could fill the spot on TBS, if that network does so. Of these performers, I think the best match to follow Conan O'Brien would be Marc Maron, bringing something like what he does with the WTF podcast to TV.
Kevin Smith or Paul F. Tompkins would be interesting choices as well. Tompkins would bring the possibility of edgy sketch comedy in the 12:30 spot, and Smith could play the raconteur role he has on his college speaking tours, only on the air, and has shown from his own podcasts and podcast appearances that he would be a good interviewer.
The writer also cites the Daily Show's John Oliver and Wyatt Cenac as possibilities, but I don't know if they're strong enough to carry a whole hour and I'd certainly miss what they do on the Daily Show. Also, I don't think Lisa Lampanelli or Lizz Winstead would command a big enough audience, and they don't seem to have the ties to Conan that Maron or others might.
There is one outcome to this slightly more subdued late night shuffling (compared to the Leno-Conan fight) that I do fear though -- and that's one name on the Huffington Post list: Reggie Watts. See this review. The fandom he gets from Brooklyn hipsters and others really baffles me. As I wrote in that review in May 2010, Watts performs an unfocused and ADD-addled combination of rap, beatboxing, odd stories and flat one-liners. Somehow, Watts is tight with Conan, who had him open on his 2010 summer tour. Watts is the one name on the list that might have an inside track just because of that, and if Conan's influence rules the day on who will follow him at 12:30, he will be making a mistake that will dent his reputation if he hands the spot to Watts.