Without trying to be pedantic, the sequel will be 'Mortal Kombat 2' as the first game was a complete reboot, it wasn't MK9. Sorry its just a pet hate of mine.
Next Mortal Kombat will be MKX. Mark my words.
-Keep the MK9 gameplay intact, just tweek the balance a bit and remove P1 advantage.
-Add new characters, but keep everyone from the previous game, Tekken style, even if they died in the previous game's story. They don't have to be in story mode.
-Bring friendships back. Babalities in 9 were great.
-Story mode shouldn't revolve around one character per chapter. Some characters in 9 got their original roles greatly reduced because they already had their chapters before their participation in some of the story events.
-Different model sizes, please. Everyone is exactly the same height. It looks stupid.
-Maybe, just maybe, bring back two fight styles for each character like in the 3D games. They pulled it in Injustice, it could work now.