Yeah, Ryan Reynolds is the ideal choice. I use to be a Dane Cook for Deadpool fan but every movie he's been in has bombed at the box-office and he himself isn't really much of a comedian. Sorry, maybe it's me, but I find him to be overrated.
Adam Baldwin is a nice idea. I think though I would want something like what Dan Castelanta did with Deadpool in the "Marvel Ultimate Alliance" videogame. Someone who can switch off being one minute serious, the next crazy, the next funny, and the next someone clearly off his medication and just ready to fire some pistols and blow **** up. If we can find a guy like that for DP than great.
I don't know who could play Cable though. I was thinking Clint Eastwood but to me he's Bruce in a "Batman Beyond" film. Bruce Willis with white hair wouldn't be a bad Cable.