I've said many times, and will say again, the one person I'd have moved heaven and earth, and sold my soul to the devil to get in the 50th, if I was in charge (or believed in souls) is William Russell as Ian Chesterton.
He's the only classic series actor/character you need.
I agree with that. The opposition of an older Ian to a much a younger Doctor would be priceless.
However, that doesn't mean I wouldn't mind more classic characters popping up!