One assumes it'll pretty much consistently be a wrestler, but considering there's a plethora of former celebrities that the WWE has worked with, they can tap into that. Off the top of my head, Kevin Federline (K-Fed) would be a great celebrity option, especially since Cena's on Raw. It was a fun angle back in the day, I thought, and a one night return would be amusing. Mr. T would be awesome, but he's pretty pissed at the WWE, apparently, so maybe not. Chuck Norris, who was a special guest ref at Survivor Series 1994 for Taker vs Yokozuna, would also be great, because who doesn't love Chuck Norris?
But that's just throwing out the viable celebrity options. I'd prefer, and think it will, consistently be a retired wrestler. Steamboat is almost a given - he's a road agent and is pretty much backstage at every Raw, so his stepping in for one week is a great idea. He's the ultimate babyface and still hugely over, so he makes a great face host. I could definitely see him getting involved with Legacy as well, since they sort of roll along the same lines as Jericho did in the Mania feud. While we're on Legacy, Dusty Rhodes is almost a given, but I imagine he'd work very differently than DiBiase did. DiBiase was sort of with Legacy, but sort of not - very much the tweener. Rhodes would have to be an all out face against Legacy, but it'd still work pretty well.
I also love the idea of injured wrestlers doing it. Edge would be great, of course, and I'm sure at some point in his recovery he will, just to remind people that's he's still out there. I can almost certainly see HBK returning on a guest host roll, then getting back onto the active roster. Matt Hardy could even do it one week, for the hell of it. He won't be out as long, but if he did it smack in the middle of his recovery it'd be just enough to keep him in the public eye and remind us about him before he returns to SmackDown.
Flair is another given, but he probably won't be terribly exciting. Some dream picks that won't happen are Bischoff or Hart - both are pretty upset with the WWE, so it won't happen. I'd probably think that Hogan will wind up doing it if it lasts that long, he's been up for one night appearances for a long time, so why not. Savage is a big maybe, but it's possible.
I think there are some locale specific ones that'll pop up sooner or later. If there's a Raw in Michigan we could definitely see Rob Van Dam do it, since he's proven he's up for one night appearances several times. The same goes for a Raw in Toronto, or Canada in general, and bringing in Trish Stratus. It might be a stretch, but Lita's done one night appearances since retirement, and she'd be a fun option.
There's a huge pool of former talent out there to tap into, and the longer they keep the guest hosting on, the more we'll see. It's a great idea to keep Raw fresh and bump ratings with returns, so I'm on board with the idea for as long as they want to keep it on.