If they were to make a movie, I hope that the series would still be as top-quality as it is already. The reason I say this is because one of my favorite childhood series, Thomas the Tank Engine, is now a shadow of what it once was, and it can all be traced to the movie Thomas and the Magic Railroad. That was where things started to go downhill in quality. Up until that point, Thomas was incredible (I still watch the episodes I grew up with sometimes just for nostalgia). After Thomas and the Magic Railroad, it just wasn't the same. In fact, a few years after the movie came out, I stopped watching Thomas entirely just because the newer ones weren't that great.
However, in spite of this, there is ONE redeeming factor that should give all us bronies hope: Lauren Faust is at the wheel. Remember the Powerpuff Girls? That show was incredible! Even though when I was younger I wasn't a fan of girl shows, I still watched it because of how "cool" I thought it was (I still like Buttercup out of the whole bunch). As many of us know, Lauren Faust did a lot of the writing behind this show.
I say all this to point out that The Powerpuff Girls kept up amazing quality in both the show and the movie that was made several years ago all the way up until the final episode sometime in 2004.
Keep up hope, all you bronies! The best from FiM is yet to come!