With Injustice 2 releasing in a couple of months, Netherrealm Studios announced a new in game currency that can be purchased with real money. Called Source Crystals, the currency let's you upgrade cosmetic items only, and will not be a pay to win type of set up.
Source Crystals will let you upgrade one of 3 items in premiere skins, shaders and transform gear. The skins are the ones similar to the deluxe edition, in which your moveset stays the same but your appearance and voice is different. This could be something like making the Flash look and sound like Reverse Flash. You can also buy shaders to change the color of items, or transform it. This is similar to other games where your stats stay the same while merely swapping out its appearance for a different item.
One other feature that Source Crystals will use is the ability to make other characters level 20, presumably with a stat boost pack, but only after you've already earned your way to level 20 with a character. This is to ensure that your progression is not cheated while playing the game according to Netherrealm. No price on bundles for Source Crystals is available yet, but it appears to function basically just like silver in Destiny. You can see the full announcement here, and Injustice 2 releases May 16th on Xbox One.