It sounds like it could be pretty amazing if done correctly. If done wrong, however, it could very well be a repeat of the previous generations.
The show has little hope of succeeding if it gets stuck with a crappy staff to get the work done. What we need are some talented writers, artists and musicians to handle the job. Since to my knowledge a season four is being worked on, Hasbro wouldn't be able to have them working on both FiM and Equestria Girls (EQG?) all at once without slowing the production time. They could split up the team and mix them in with fresh meat, but that may or may not alter the overall quality of FiM. And in all honesty, I'd like to have more enjoyable pastel ponies plus distasteful EQG rather than sub-par pastel ponies and a possibly shitty run of the latter.
So, indirectly, it may very well waver my opinion on FiM. If they choose to split up staff or have the team work on both FiM and EQG, it'd effect the quality of the show and thus make me like it less. This is just my opinion though, It could end up not effecting our beloved program in the slightest. When Lauren left after season one, the quality remained just as good, so who knows.