I must agree with you there. The overall fan base is rather big now, but the thing is that, I doubt that The MLP movie would get lots of views in cinemas (and make lots of money for the film makers and the cinemas), because I think about 35% of the whole MLP community would go to cinemas and pay to watch the film, and the other 65% would just pirate it. Well, it's only my opinion, that I made looking through the internet and my country Facebook discussions. What I'm trying to say is, that the film wouldn't be so successful like cars 2. I think it will be something like The fox And The hound 2 (that animated film didn't get a lot of views, so disney abandoned the idea of making the 3rd part)
Anyways, It would be awesome to see that film on the cinema, and I bet it would have a great plot (as the whole MLP:FiM show)
P.S. Sorry, English isn't my native language, so there might be some mistakes, and you probably won't understand a thing that I wrote. edit: so don't be mad at me.