At the very least, they could add more of Sue's powers to Kl'rt. Give him the ability to turn Invisible briefly or at least pummel the opponent with Forcefields.
IIRC Super-Skrull had a lot more of Invisible Woman's powers originally, like some parts of his moves would turn him invisible for part of the animations, but it turned out to be too much
juice.
Otherwise, yeah. Never cared for F4 as a kid, still don't now. This is even in spite of a relatively cool concept behind the team, how they got their powers, and the relatively novel powers themselves. They just never gripped me. I'm with Dahbomb, all the periphery characters like Silver Surfer, Galactus, and Dr. Doom are actually interesting in comparison. That's why I'm with Super-Skrull as the solution instead of including the whole team.
Might seem extreme, but I wouldn't even mind if the game was delayed to add X-Men into the base roster. I doubt it'll happen, but I really feel like this game starting off on the right track is important.
This is so important. The game needs to be feature complete, and not just because Injustice 2 is one of the most solid fighting game packages, if not the, of all time. If this game doesn't have all of it's faculties on Day 1, sales will suffer, and so will post-launch support. And I want post-launch support to stretch out way more than one or two seasons, I want this to be a platform that thrives.