Maybe this is a wild take, but I would rather Trixie had become the new member instead of Starlight Glimmer.
Here's an idea: imagine an AU where Trixie didn't get reformed after “Magic Duel” (season 3).
After Twilight became an alicorn (season 3), Trixie gets super jealous. Trixie has always secretly resented her cutie mark, because she believed her special talent was only “entertainment magic” than hard-core magic like Twilight. Trixie knows that the only reason Twilight got where she was was due to her cutie mark being magic itself, and Trixie begins to get fed up with how a pony's career is predetermined for them, and ponies judge each other by their cutie marks alone.
Trixie does research on the most powerful magic users throughout pony history, from unicorns, pegasi, and earth ponies.
In canon, Starlight Glimmer makes up a story about finding an enchanted staff created by Mage Meadowbrook (an earth pony Pillar of Equestria), but in reality it was her own magic.
In this AU, the staff is really magical, since Trixie (at this point in the story) doesn't have powerful magic. But I don't see why Mage, a down-to-earth… earth pony, would create such a thing. I can see Star Swirl, the mad scientist unicorn creating that (either accidentally or on purpose). And since he already dabbles in cutie mark magic like his unfinished spell (the same spell that turned Twilight into an alicorn), that would work. And then Trixie acquires it and creates her cult…
The same sequence of events plays out, more or less. So after all that, Twilight takes Trixie in as her pupil. At first they're still a bit sassy with each other, but eventually they soften.
I find Trixie being in Starlight's place a lot more impactful, since Trixie and Twilight had a continuous rivalry since season 1. Also, unlike Starlight Glimmer, Trixie's backstory/motivation isn't garbage and she wouldn't be given special treatment by the narrative.
And Trixie being Sunburst's childhood friend still fits into the “day/night” pattern I pointed out before. Not 100%, since her name doesn't reflect light from the night, but at least she's still night themed.
I really wish Starlight Glimmer was a better written character. There are aspects of Starlight's character that I like, but in general the writers fucked up. She's a lot better in my head tho.











