I bet you I can sort of predict what might happen in the next Equestria Girls film...

Since Starlight is probably going to stay with the Mane Six (even though a part of me hopes she doesn't and they forget she ever existed), she's also going to make an appearance in the next Equestria Girls film, whatever that may be.

I think she'll work like Sunset does. There won't be a human world version of her, and Twilight will bring her through the portal to meet the human world versions of her friends (and herself... I'm not sure how that will play out unless they completely abandon pony world Twilight and choose to focus on human world Twilight instead) who will make friends with her, and blah blah. Exactly like Sunset.

If there was a human world Starlight, she'd probably be older than the rest of the Mane Six (although I bet pony world Starlight is older anyway) and be the mayor of Our Town, which Sunset would find out about, then she'd write to pony world Twilight about asking her for help. Pony world Twilight would write back saying she knew how to deal with Starlight, and then the story would stem from there.

Seriously, human world Twilight joining CHS was a weird move which could possibly end the Equestria Girls franchise altogether, although they'll probably find a way around it.