On most games that have this, 'character customization' equipment activities, usually all the devs put out DLC packs to get more money. I can easily see them pulling a DoA and releasing "Christmas Pack 2016 - 2.99" or something like that. Just an example.
Seeing that they already seem a bit annoying with the Micro trans from MKX, I'm sure they're not going to shy from adding tons of it to Injustice as well. I mean what game doesn't have micro transactions anymore? Injustice 2 will get just as pricey as MKX. But nowadays I learned not to have an issue with it. If I like, I buy. If it bothers me, I don't.
Much rather wait til an Injustice Complete Edition comes out if it were to come to equipment packs for too much money though.