I came up with this theory after reading this
https://twitter.com/TheBiggestJim/status/916109643570544640

I know this was a bit disproven since discords head was seen in the animated credits of the movie and those who have seen the leaked episodes can debunk this, this was a theory after all.

My theory was that if this takes place between 7 and 8, what if something happened in the season 7 finale where Discord is no longer with the ponies? What if to stop the pony of shadows he had to do a sacrifice along with the legendary ponies?

What do you think and like it said it is probably debunked.

Ps if you seen the episodes and if I was close please don't spoil anything for those who want to wait

No, I think the only reason they didn't include Discord in the movie is because if they did, the movie wouldn't even be a movie and the whole plot would get screwed up.

Discord is simply way too OP for dealing with threats to Equestria. Just a snap of his eagle talon, poof, threat gets turned into entertainment for him.

Besides that, if Discord got involved, he'd have to fix Tempest's horn. He'd have no excuse not to.