There is an actual MLP movie that's supposed to come out in 2017.
I will say that an Equestria Girls 3 could be interesting if the actual human Twilight is introduced. As far the 100th episode goes, however, I'm a bit torn about it. On the one hand, it could provide some interesting characterization and possibly tons of butthurt(imagine how angry bronies might get when it turns out DJ Pon 3 isn't into dubstep or Lyra doesn't have a fetish for humans). On the other hand though, it could also be the ultimate in fan pandering. Do little kids even know or care who characters like Derpy or Bon Bon are? I do hope that it is a good episode nonetheless. I just don't want to see this show become The Brony Quarter Hour.