I don’t think Morrison would be suited to 2000 AD these days and Millar never was. Ennis is someone I’m happy to see back doing things for Rebellion, but he seems happier with his war stories, so that works. Of the remainder, Milligan has written for 2000 AD fairly recently anyway, which kind of rules him out, surely? Hogan too.
Still, if we’re talking more regular appearances, I’d have to go for Ewing. Get him alternating with Niemand on Dredd and bring some serious consistency to the world (with other writers chipping in as and when); and if he’s there “on a more permanent basis”, I’d love to see an original strip or two from him as well.
Runner-up for me would be Hogan. Resident Alien was a wonderful strip, which I hope gets HC treatment. I’d love to see how his more thoughtful sensibilities could work in the modern 2000 AD. (Double points if Steve Parkhouse comes along for the ride.)