There are more ways to celebrate than having returning Doctors, or does fandom lack that kind of imagination?
It's nothing to do with "lack of imagination". It's to do with Respect, something Moffat seems to lack towards the people without whom he wouldn't have a series to be running into the ground through his incompetence.
This is the last chance to bring any/all of the Original Series Doctors together to celebrate their contribution. Having paintings of them appear, which seems to be what's going to happen or sticking Tom Baker's scarf on a Guest Star is worse
than simply pretending the series didn't exist before 2005.