I know I mentioned that my theories about Sombra being a possible cause of Luna's transformation have the possibility of being discredited, but I'm choosing to think positively about the canon. If you really like your idea, make it an AU in itself. It might need a little alteration to make a really good universe, but it can still work.
The way I see it, there are two major ways that the show could kibosh Dislestia. 1: It turns out that Discord is Celestia's father/brother/mother/uncle/janitor. 2: (Is a big fear of mine): If the episode that DeLancie says he's doing happens to be a flashback episode. Now, it's a bit difficult fitting in the entire childhood of Celestia and Luna within 23 minutes, so it makes the whole *Discord was friends with them* theory a bit harder to pull off. My fear is that the flashback will be an episode that's more about Celestia and Luna discovering the elements, thereby casting Discord to the role of "BADGUY WE GOTTA STOP".
I've talked this over with my friends, and we agree that the best route to take with Discord is to have him return in a single episode appearance in which he loses his powers. It would allow him the opportunity to interact with the mane cast a bit more, and provide for some bickering with Celestia, and maybe for some making fun of Luna. Really, I think they should just leave any specifics open to interpretation, and just hint at the connection between Discord and Celestia.
But seriously, Dislestia is best ship, and I pray to god that the writers take note of it. Including it adds another element to both of these characters, which I believe would be an excellent thing. Especially, in the case of Discord. It would finally put the "DISCORD IS COMPLETE MONSTER" arguments to rest. And believe me, you wouldn't believe how many times I've heard that point stated.
Anyway, anyone who isn't the staff of FIM, their stories will always not go as planned, since no one but say the staff know what to predict and expect. Sometimes, ya just gotta go with your things and ideas for stories, art, or even comics. Trust me, it's hard for me, I am so compulsive it's not funny To be honest, Sombra really works into my favor of my big FIM Dislestia-esque story. May actually post a possible spoiler fanart as to how or why, soon... or eventually.