one of the lines i have for my (theorectical) Four Doctors 50th anniversary story is Ten claiming to be 905 and Eight saying, 'Hold on, I'm over a thousand' to which Eleven replies, 'I lied about my age a lot when I was him.'
That's be fantastic...just the whole notion of McGann, Eccleston, Tennant and Smith on some adventure together is awesome...heck, toss McCoy in there if he's still around too...