It's hard to see Cleese being invited back. His take was much too light-hearted and frivolous for the Craig series, whereas Dench fits in as well here as she did in the Brosnan era.
I said some time ago that I'd like to see someone in the vein of a younger Michael Caine or Edward Woodward (and not that plank of a son of his!). An armourer, someone with a touch of the Sgt Major about him. Also, in the first few movies, Q wasn't an awful lot older than 007 and I'd rather go back to that than the more avuncular character of the Dalton & Brosnan eras.
Must admit, I can't think of anyone who really fits the bill. Eccleston isn't quite who I had in mind, but I could see him working. He can alternate quite easily between grumpy ('Put that DOWN 007!') and giddy (as when showing off his latest inventions). I've a vague recollection of reading that he and Craig didn't get along when making Our Friends In The North in the 1990s, but maybe a bit of friction would be what they need.