Cleese wasn't joking, he was seriously pissed about not being included in CR. Even though he seems pretty mercenary, I don't think they could make it worth his while.
I actually think they need a non-actor in the role. The guy who makes the high-tech vacuum cleaners and does his own commercials is ideal for Q. His voice is interesting enough (which is probably more important than acting talent for THAT role) and he could probably just be himself.
I'm more interested in them casting Moneypenny or Mary Goodnight. Could be a past Craig costar ... Greta Scachi would have been a great Bond girl in the Dalton era, Catherine McCormack in the Bros era, either would be great Moneypennys now (I'd have killed to see mcCormack play Vesper Lynd, given how well she worked with Brosnan in TAILOR OF PANAMA.)