I saw one report, I can't find it now, that said that not only would Ferguson not be a likely replacement for Letterman he apparently is on record as saying he doesn't want it. He's perfectly happy at 12:30 by all reports.
Colbert would be interesting, but in a negative way: there's no way he could get away with what makes him so brilliant if he were on CBS, and losing the character "Stephen Colbert" on CC to gain regular Stephen Colbert on CBS would be a huge loss to my eyes.
Drew Carey would be a safe, boring pick. CBS is known for safe and boring, though. That's kind of why I think what Ferguson has done (under the secure umbrella of Letterman's production company) by undoing and redoing the talk show format is so wonderful (and so likely not to succeed at 11:30 like Fallon clearly was determined to succeed.)
There's gonna be a lot of pressure, especially after the Seth Meyers pick at Late Night, to have a non-white, non-male host. Late night is rather homogenous...
Words don't deserve that kind of malarkey. They're innocent, neutral, precise, standing for this, describing that, meaning the other, so if you look after them you can build bridges across incomprehension and chaos. But when they get their corners knocked off, they're no good anymore…I don't think writers are sacred, but words are. They deserve respect. If you get the right ones in the right order, you can nudge the world a little.