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).
I believe you're not the only one. In fact, if everyone wanting this would write a letter to Hasbro, this would totally be in Season 4, just like all those fan letters made the second and third season of Star Trek happen back in the old days.
Luna: Trixie, heed my words! Thy powers are not what makes thee, it is thine choices that decides who you are... You are MY student as she is my sister's, and so long as you remain my student I will teach thee all I can, but you must trust in me... Trixie: But-... Luna: No buts, now pick yourself up and follow... There is much for you to learn and the night will not last much longer... It will be time for me to lower the moon shortly...
Using someone else's character might be a bit tricky, considering a number of factors. One of which would be, is the creator picky about how their character is depicted? The MLP fandom is massive, I prefer to look at Trixie as somepony who has potential to be good but has made some bad decisions. Princess Luna taking an apprentice seems almost necessary to me, so why not someone who's been down and out like she has? I need to be able to gauge a character within their own element before I can create my own dialogue for them...
Garble only served as a temporary antagonist, someone that Spike had to try to overcome on his own. To that end, once his role was complete we never saw him again. Perhaps my statement was unfair, but Garble doesn't show any signs of having any character development. In general, most bullies are the way they are because of bad circumstances at home or within their families. However, there are some that are bullies for the plain and simple fact that they have a warped sense of "fun", and get their kicks out of inflicting torment on others. I hate to say it, but Garble comes across to me as the latter kind of bully.
Luna: Once I truly believed my sister was loved and praised more than I... I grew resentful of her and the daylight she would bring and fell to darkness... That is why I tell you now, regardless of what may be, banish those feelings of resentment from your heart... Fill your heart only with love and joy... Trixie: Princess... Luna: I will not have my student repeating my mistakes! I will not have you become the next Nightmare Moon... Does thee understand? Trixie: Yes ma'am...
Post-Alicorn Amulet Trixie, we've got some serious commonality, not just their hamminess.
My headcanon is that the Alicorn Amulet doesn't grant knowledge of a spell, merely power and control to cast what you know at incredible levels. If that's the case, Trixie's demonstrated that she understands conjuration (the pie attack, clothes beam), transmutation (Rainbow Dash's wing, her carriage), weather magic (that big snow drop), shield spells (encased the entire town in a shield), agony spells (without the amulet, Trixie can't get it to more than tickle, but overstimulation of any kind can be unpleasant), and age magic (Snips and Snails).
Plus, with a wand Cutie Mark, you could upgrade Trixie's power fairly easily by having her be reliant on focus items.
Trixie: I will always be second to that Twilight Sparkle Luna: Now, now, I myself am second to my sister, but I Lerner to not to let it bring me down, otherwise I would become that Nightmare Moon, but I will not let that happen again, nor should that happen to any pony else, if you wish I can help you reach your full potently Trixie: Are... are you... saying... you want me... to be your... Luna: I would be honored to have you as an apprentice