Hmm. I rather enjoyed a certain Star Trek/Here Come the Brides crossover.
And I seem to recall that in some Diane Duane novel, there was a cameo appearance by what appeared to be an old Dr. Who video.
Now, of course, if somebody else known only as "The Doctor" were to display a large "H" on his forehead, to indicate that he's a hologram, then we'd have a crossover with yet another franchise.