This is the official cast that I've found so far:

Lui Kang.. Robin Shou

Linden Ashby.. Johnny Cage ( same dude from first movie)

Kitana.. Talisa Soto ( same actress )

Mileena.. Dana Hee

Rayden.. Christopher Lambert

Tanya.. Stacey Dash (girl on clueless)

Jax.. Lynn Red Williams ( MK:A actor )

Scorpion.. Chris Cassamassa ( MKC and MK Actor )

Kung Lao.. Paolo Montalban

Quan Chi.. Adoni Maropis

Those are the Confirmed characters at this point.
Characters that will return or have a good chance.

Sub Zero - Definite to return.Not sure of Actor.

Shang Tsung - Confirmed.Not sure of the actor though. Hopefully Bruce Locke.

Sonya - Confirmed, 95% Chance of Sandra Hess playing her again.

Sindel - This quote came from the producers, "You Thought Sindel Was Powerful in the Second movie... Just you Wait!" so yea probably going to return. Could be the same actress from MK:A, they are unsure.

Shinnok - Confirmed, not sure of actor.

Frost - 85% chance, most likely actress is Sung Hi Lee ( she was also in MKC once or twice. )

Reptile and Shao Kahn have a 50% chance or less than being in the movie.

Filming begins in November due to script changes. Like I said they're trying to avoid another disaster of MK:A, according to producers.

From what it sounds like from the Hollywood stock exchange, it seems like the story is going to follow mostly the story of MK:Mythologies.Although I highly doubt this, here is the link.

http://movies.hsx.com/servlet/SecurityDetail?symbol=MK3

Although if this is true, it will probably play a part in just the beginning of the movie.