I don't know the complete ins and outs of video game certification, but I'm guessing that if the next Mortal Kombat game were to be deemed too violent for an M rating, it has a few options:
- It gets bumped up to an AO (Adults only) rating, which is used for games with very strong sexual or violent scenes.
- The ESRB may deny certification to the game completely, essentially meaning it's banned from being sold in the US. (Though don't worry, the chances of this happening are very, very, VERY slim.)
- The developers may be instructed to reduce the level of violence in the game, otherwise the above may occur. (This is also very unlikely.)
In my honest opinion, I don't think the ESRB will find the next Mortal Kombat game too violent for an M rating, as AO ratings and banned games in the US are very rare.
Of course, I can't say the same for countries like Australia, with very strict certification rules. Don't expect the next Mortal Kombat game to be released there.