I really hope we will see more of McGann in the future. Also, I hope we, in Series 8, get a season long villain that is menacing... and STAYS menacing (looking at you, Silence). I feel the plot for next season, should have constant reminders that the villain is still there, as well as gradually spacing out plot revelations (Series 6's problem, IMO, was that it hoisted it's plot points on the viewer in quick succession with several filler episodes in between).
I also wish we could get some Colin Baker and McCoy cameos in the new series. Sylvester can still probably pull off the roll convincingly, even though he is a little older. If Capaldi's Doctor is darker, it would be nice to see two dark doctors meeting up.
As for Colin Baker, I don't think we'll be seeing him again as the Doctor. He has changed far too much to pull off the role convincingly. However, there is always Baker's original role in the show.
If they bring the Time Lords back soon, I would hope to see Maxil (played by Baker) as well as Romana (played by Ward, and if Big Finish hasn't done it yet, regenerating into Juliet Landau). One of my biggest disappointments with the 50th Anniversary was how many close chances they had of referencing the Classic Series but missed. I wanted to see William Russell as Ian, as the first human companion ever (along with Barbera), I would have liked to see Ian somewhere in the 50th, since they do reference him. Also, the general could have been easily retooled into GENERAL Maxil, played by an older Baker, fixing the age problem.