The only characters named so far are Jack and Gwen. My guesses are usually way off the mark, but I suspect that Mickey may end up as a team member, with Martha as an occasional visitor, eventually marrying Mickey and both leaving on idealogical grounds. Possibly one or two of the characters from Children of Earth?
I doubt that Ianto will return from the grave, despite being in the photo that illustrates the article I linked to - they already did that with Owen. And I REALLY hope they don't try to bring back Tosh or Owen yet again.
Who else? Not Donna, Rose, etc. Probably not Doctor 11 any time soon. And I doubt Sarah Jane, Luke, and co., she'd go to GREAT lengths to keep Capt. Jack out of their lives.
Captain John is an obvious possibility - he's presumably still around somewhere in time, currently James Marsters is in Caprica but that doesn't look to be a very important role so maybe he can find time to do other things.
Anyway, it will hopefully be interesting.