Lol well duude by now i can tell you that there are many ways to unlock a character costume:

1) Beating the arcade tower like in MK9 and Injustice}

2) Unlocking some random stuff in the kript: In MK9 you can unlock your alt costume by beating the tower or unlock it purchasing in the krypt.

NOTE: Since in MKX some characters have more than 2 costumes, one could be onlocked in the arcade tower and another in the kript.

3) DLC SKINS: this skins are purchased online. They come normaly as a whole pack (3 skins, each one for a diferent character) and this could also come with another things like characters. it depends of what you are buying.

4) CROSSOVER PLATAFOMR UNLOCKS. Since injustice you could unlock certain costumes only by playing the mobile version fo the game. You must link your mobile version with yout console account, then by completing challengesin your mobile, you unlock rewards in your console version. (This is how you unlocked batman beyond costumen in injustice) 5) RAMDOM IDEAS FROM THE COMPANY! now the companias have a lot of marketing strategies to support certan things, for example, Raiden future costume (tron costume) is one of them. This depends of the ideas that a company has to promote certain things and this is not very likely to hapend with more characters, but we will have to wait and see what happends.