Everybody is just assuming it's Michael Cera, but what if it isn't? I've seen some quotes from him that make me think it might be, but he's never explicitly said he wouldn't do it (he would be stupid not to do it, sure he's a bigger star now but he wouldn't be if it wasn't for AD and it's not like he's a household name yet or anything).
I'm more inclined to think it's Portia that's holding out. As someone else mentioned, other than marrying Ellen she's been keeping out of the public eye since AD, and wasn't there some unpleasantness between her and Mitch at some point?
Which cast members are confirmed at this point? Obviously from their quotes in interviews Jason Bateman, David Cross, Jeffrey Tambor, Alia Shawkat, and Tony Hale are all in. Do we know anything about the other cast members? There's not any possibility that Will Arnett is holding out, is there? He's a bigger name now than he used to be and he has a newborn baby (I mean, I know he's been talking about the movie, but was that before they had the baby - has he said anything about it since then?). Or what about Jessica Walter? She's on 90210 now and I seem to remember reading somewhere that she didn't get along well with everybody either.
I hope they get everybody on board, but if they can't I would be least disappointed if it was Portia. I think with all of the crazy stuff that happened at the end of the show, Lindsay would be the easiest character to write out. Lindsay's my least favorite character anyway. Of course, saying that Lindsay is my least favorite AD character is like saying that dark chocolate is my least favorite kind of chocolate. I'd definitely be sad if they couldn't get everybody.
Buster: Mom is becoming a little controlling.
Michael: What tipped you off? When she locked you out on the balcony again?
Buster: That was half my fault. I thought I saw a graham cracker out there.
Michael: You baited the balcony?
Lucille: Prove it.