IMO, they'd have to toughen him up a lot (I like John). I know you're probably not advocating for a 100% clone-Rick-swap-in, but I think anything less just wouldn't feel right for a character in that role, if that makes sense.
He is one of the ones I wouldn't mind seeing cross over though. Him, Dwight and Morgan, and maybe Joan and Al. Alicia has been growing on me. Tired of Daniel and Strand's characters; Wouldn't necessarily want to see them in TWD.

 

I like that idea, established side show/spin off characters with two from main show that got to develop their character in the FTWD jumping over to TWD to fill it with some interesting new characters!