It seems Patrick Stewart is a long-time fan of Doctor Who and expressed his desire to be in an episode in a recent interview:
“I have been (a fan) actually yes, and deeply dismayed that I was never asked to be in it! I think it’s a terrific series,” Stewart said.
Stewart is currently set to appear opposite current Doctor David Tennant in Hamlet with the Royal Shakespeare Company next year; Tennant will play the title role and Stewart will play Hamlet’s uncle, Claudius.
We’ve already had Giles (Anthony Stewart Head) from Buffy the Vampire Slayer in a second season episode so why not one of the captains of the Enterprise? I think he’d make an excellent villain.