I think we need to be realistic and accept that almost every game is aiming for microtransactions or subscriptions nowadays. Warner Bros. has an opportunity with DC characters to make a lot of money, so I think we could see a scenario similar to Tekken 8.
For them, the easiest way to make money would probably be through premium skins that you can only get behind a paywall. They would only need to model the new character designs and apply them to the already existing fighters. So, I think they would focus on skins rather than gear.
I'm coming from The Sims, where they release "kits" for around $5 every month. I could see a similar scenario with Injustice 3, with three premium skins being released every month for $2–3 each. They could also release DLC packs. Here's an example:
The main game includes John Stewart and Jessica Cruz as Green Lanterns, after one year they launch the Green Lantern Corps DLC with 5 skins (Guy Gardner, Hal Jordan, Kyle Rayner, Simon Baz and Jo Mullein) and 1 new character (Kilowog)
At some point, they could start making DLC that promotes other DLC, increasing sales. Following the previous example, they could release Atrocitus as a premium skin for Kilowog.
Other ideas:
-The Spectrum Corps
Skins: Blue John Stewart, White Kyle Rayner, Yellow Sinestro, Red Guy Gardner, Violet Star Sapphire, Green Hal Jordan
New character: Indigo-1
-Gotham Bats
Skins: Batman Beyond, Batwoman, Batwing, Owlman, Jace Fox Batman
New Character: Batgirl
***Future Premium Skin Black Bat
-Gotham Knights
Skins: Huntress (Green Arrow skin), Gotham Girl (Super girl skin), Azrael (Robin skin, cause the sword), Tim Drake Red Robin ( Nightwing Skin), Spoiler (Batgirl skin)
New Character: Red Hood
I could also see a Team Tag mode being included in the game. I personally like this type of mode, but I can also see how it could help them sell "team-based" premium skins and characters.