It occurred to me this morning that Colin Baker could've conceivably had a cameo in TDotD as an aged Commander Maxil.
That idea occurred to me back when we first started getting hints that the Time War would be featured in the special. Yes, it would have been a great way to bring back a former Doctor without having to worry about the age thing. But, then, logically, with a war looming, wouldn't Maxil have wanted to regenerate into a younger body so he could be in his prime to fight? Hell, by this point, Maxil could have been in enough battles to actually end up going through a few regenerations. Hell, the general wasn't named, so maybe he was Maxil?