Not sure how old this topic is but I wanted to comment on this as yesterday I finally got to watch the MLP Movie and I found this aspect a little disappointing about it. Fair warning, I'm assuming everyone has seen up to Season 7's finale (and I'll be mentioning something on Season 8 ep 1) and all the Equitria girls movies and the 3 (or 4) specials (particularly Mirror Magic)
With the spoiler warning out of the way, first of all I wanted to address something from an earlier post. Granted it happened most likely before Season 8 Episode 1 came out but I still wanted to make note. But in that particular episode it made it pretty clear that the Movie takes place between Season 7 and Season 8.
Now on to the main topic at hand. You see, I honestly was a bit disappointed in Starlight only getting a brief cameo in the story as she's currently my favorite character. Admittedly, I'm perfectly fine with her taking a backseat and not even going on the main quest with the others just what bugs me is the fact that it makes no sense as to why she didn't get involved in some way. I mean they could of easily added a background scene or something of her getting captured or something but in a way I was thinking of it and tbh I think they should of done one of the following imo.
1) Removed her cameo entirely or at least shifted her cameo to after the day was saved. Because putting it where they did I just find caused more issue than it helped as it doesn't make any sense that a powerful pony such as Starlight didn't get involved at all. Not to mention her friends were in danger, so seriously. With this one, you could at the VERY least assume that Starlight was simply not there. Like I'll admit, I seen Equistria Girls special: Mirror Magic before I seen the movie and from what Starlight mentioned, I assumed that it took place during the movie. But for those who don't know. Starlight takes a trip to the EG dimension in that special and based on what Starlight said about "The girls are away solving an issue" personally I think it would of worked well for why Starlight wasn't there and that's what I had assumed. But sadly if you go by more what she said and then the movie, well she's obviously not referring to the movie. (unless she was but if so she did a really bad job at referring to it.) Plus at the end of Mirror Magic, she was told she could stay a few more days, which while it's possible there was a few days time skip to reget the friendship festival back in place, it was assumed it took a few hours. But alas, it wouldn't work but still you could assume that Starlight was away and thus simply didn't know what was going on... had they removed/moved her cameo.
2) Now, I get the fact that adjusting the movie may have been difficult as the story was in more production from before Starlight really became a good pony. But there was a spot in the movie that would of not only given her a 'role' in the movie but also explain why she wasn't more of a help. And it wouldn't require any real adjustments to the script. Anyways, in the scene where Storm King's minions first show up to the festival and started to capture the Princess'. Now when the 'magic ball' (dunno it's name) was going for Twilight, Twilight was saved at the last moment by Dopey (that's her name right?). Personally, this would of been a great place to add Starlight. As in have it where it was Starlight who saved Twilight instead of Dopey. Wouldn't of been to much of an adjustment, she didn't even have to say anything tbh, and the psycho-out (audience and tempest thinking she got Twilight) didn't even last very long, as in she realized it pretty fast. I think it would of been funny for a one-liner similar to in the episode Triple Threat where Ember kept confusing Starlight with Twilight (you're a purple pony with purple hair and with star cutie marks with shiny things ) But not required of course . Anyways, this would of been a good spot to of added Starlight without even needing really any adjustment to the script and small adjustment to the animation.
So yeah, that's my oppinions of what they should of done. But I would also like to make another note of why the Pillar's of Light didn't do anything, (aside for them not being introduced until literally the last episode before the movie) Simply put, they were traveling and thus not even there at the festival to know what happened. Not to mention in the Movie's time.. the events lasted I'd say a maximum of 48 hours (if that) so while StarSwirl could of just teleported there, really it's more than possible they didn't even hear about it until it was already over. You could of even made the same excuse for Starlight... if she wasn't literally there... which if the EG special WAS her reasoning, could of simply been she was told off camera or something to head back to Ponyville to deliver the journal. And she can also teleport long distances so wouldn't of been an issue there.