Injustice 3: 10 Supervillains NetherRealm Must Include
4. Amazo
Despite having been absolutely annihilated by Supergirl in the Injustice 2 comic, re-introducing this character into the Injustice-verse would be a great way of providing a challenge to Batman's new Justice League.
If you didn't know already, Amazo is an android whose ability is to absorb the powers of others and use them for his own, usually more villainous, schemes. The only slight problem with the character is his design, with him looking quite goofy, but not in the same, enjoyable, way that the likes of the aforementioned Kite Man is. Consequently, a redesign should be top of NetherRealm's priorities.
So, if executed correctly, this character would provide a superb amalgamation of the game's best move-sets (hopefully not in an overpowered way) that would be insanely fun to use, not to mention visually brilliant.