This pretty much sums up why I stayed in the Brony fandom. I was debating leaving after the 1st season. Then I heard Q... I mean Discord as a fan from when TNG was new, John De Lancie being on MLP was a sign to stick around. Now if they somehow got Patrick Stewart I'd be in fandom heaven!
You wouldn't even need to have Sir Patrick Stewart* do much, just a few lines as some kind of "old mentor" character (an old adviser of Celestia's, retired/old military officer in the Royal Guard, even a mane character's grandparent). You could even do him in a villain role (say, a pegasus "warlord" intent on restoring the "old glory" of their militant past before Equestria was founded).
*Yes, the "Sir" belongs there as the proper title; he was knighted 4-5 years ago.
Captain Sparkle, I'm seeing a flock of dolphins floating near our starboard. There's a planet that appears to be made of, is that tofu?, Omg it is. Also, there's a giant rubber ducky that's coming right at us! EVASIVE MANUEVERS!!