Emma Watson can voice as twilight sparkle

She'd be a good pick.

"Do a barrel roll"-Peppy Hare

Although Emma isn't a bad choice by all means, Tara Strong is still reprising her role as Twilight for the movie; says so on the IMDB front page of the movie's cast listing.

Nothing against Emma Watson, but why would the producers even think about replacing Tara Strong as the voice of Twilight Sparkle? Strong is the perfect voice for her, and she has not grown tired of it.
Possibly in the next generation of MLP, someone else will do Twilight Sparkle's voice, but for now it should definitely stay as Tara Strong.

No not gonna happen. Tara Strong is strongly associated with the character and also she is a great voice actress. So they won't replace her with a celebrity. Fans would rebel if something like this happend.

Lots of people would be capable of doing the voice of Twilight, but Tara Strong has already established herself as the voice of Twilight. I see no reason they would change that.

If it ain't broke don't fix it.

Either Benedict Cumberbatch, James McAvoy or Michael Fassbender could be Discord.

Aerosmith watches My Little Pony: Friendship Is Magic

Twilight Sparkle's character belongs to Tara Strong. To turn around and give it to another voice actress would be a slap in the face to Strong. No disrespect intended towards Emma Watson, but it would be wrong to change Twilight's voice actress now, unless Strong suddenly decides that she just doesn't want to do it anymore. Also I think the MLPFIM fanbase would be totally outraged if Watson was suddenly given the job over Strong.
That being said, I say this with no intention to insult anyone.... you are all entitled to your beliefs and opinions. But if anyone out there feels its time for a change, then please STFU and stop suggesting that someone else do the job. As far as I, or any other fan is concerned, the job is Tara Strong's for as long as she wants it, and the same goes for all the other Mane 6 ponies and Spike and the voice actresses that do them as well.