It will be two hours about the origin of Mustang's moustache! With an hour and a half only consisting of zooming in and out onto the object in question. That's the reason they added it!~
I think crossovers between the first series and the manga is more fanfiction realm than a good idea for a movie. Would demand too much knowledge and focus from the audience ... and honestly even a recap film would be better
A cool thing would be a little story set after the end of the series, written by our favorite cow Arakawa, illuminating the lifes of the characters a bit. Even so, it would kill the mood set by the last chapter and the "they move forward"-feel quite a bit.
A retelling of the Ishval arc would be very awesome, but I doubt we'll get that.