I won't be surprised with either scenario, Boon usually trolls but almost all of his trolling tweets lead to something, so even when he trolls it can lead to something legit. So this might be legit.
Also regarding what could be the in KP3, I think there will be a 4th guest character. And I think it's time to finally share my wish:
I want Shredder in IJ2. Yes, I said that. I want Shredder, or any other TMNT char in IJ2. A Batman/TMNT crossover was a thing (I read it, it was good read), so they can put a TMNT char in IJ2. Spawn should be in MK11 due to his dark aura, but the TMNT, while can be dark, fit the DC universe more then Spawn. Also just imagine all of the cool possibilities for Shredder with the Gear system, how many cool armor pieces he can get. If Shredder does make the cut, I will main both him and Sub-Zero (and Starfire of course).
MKX had 5 MK chars (Triborg, Goro, Bo, Tremor and Tanya) and only guest chars (LF, Jason, Predator and Alien). So it is 5 to 4. That's not "mostly guest characters" at all. And even if there is a 4th guest char for IJ2, that will still be 6 to 4, as Darkseid is also included.
Except the fact that not everyone DC char can join the DLC roster just because they are from DC. I've said this before and I will say it again. Both Raiden and Hellboy are two iconic chars from their respective franchises and some DC chars will be way more random then them. Also considering how the IJ2 story mode ended, I bet that NRS/WB will save the more popular DC chars that could've been DLC for IJ2 for the core roster of Injustice 3 when that game comes out.
Lol these cars aren't jobbers at all, and MK is dark just as the DC Universe. Hell it's a lot gorier then DC, there you go.