If there's one criticism you can't levy against Mortal Kombat 1, it's the lack of costumes, or their variety.
Just recently, there was a fresh preview of the Mortal Kombat II movie, which is coming later this year, and NRS has already announced that some of the costumes from said movie will appear in MK1.
So far only 3 costumes have been confirmed, you can get a better look at them on their respective Steam announcement pages:
Unlike some of the other costumes, like the ones based on Paul W. S. Anderson's adaptation, it seems like these ones are strictly limited to clothing, and do not change the characters themselves, as evident by Johnny Cage not looking like Karl Urban.