I'm currently working on a LunaxTrixie fic and was looking at this as a possible cover art. Would you mind? Especially since the focus of the fic is on them bonding on that understanding of being in somepony else's shadow.
Not only Luna would teach things to Trixie, but teaching her would allow Luna to learn things as well. Also, after watching season 3, another character that should be brought into this is Lightning Dust... I have this picture in my head that after she gets thrown out of the Wonderbolts, she becomes miserable but ends befriending Trixie.
If Twilight is Celestia's best student, why can't Luna have one? And yes, Trixie would be the best apprentice for Luna. Both of them could learn a lot from each other, they complement each others character in an almost perfect way.
It's rumored Trixie is going to reappear as an ally in Season 4, so maybe? It would certainly be a good idea; after all, I don't think Celestia didn't say Luna couldn't. Plus both Trixie and Luna have "larger-than-life" personas (albeit for different reasons).