Should Have Been Done As A Trilogy, Same Actors, Director etc..
If I had been in charge of the franchise I would had Mortal Kombat done as a 3 movie deal. Signed the actors on contracts for the sequels, and had the script worked to make it a trilogy.
That way they could have spread the characters out better and given them a little more substance.
I would have loved to have seen the story of Mileena being explored more as Kitana's evil cloned twin sister created by Shao-Khan.
Some exploration of the robot ninja's would have been great, there is so much in the Mortal Kombat series. that could have been explored, however it just wasn't possible in two movie's.
I think had they gone the route of using the same actors, same director, same team of special effects artists. From the first movie all the way through to the 3rd film. It could have been a good trilogy I think, though I would have left the 3rd one open for a potential 4th film, if the 2nd and 3rd did well.