I have a far-fetched theory but hear me out: Cal Kestis becomes Ren.
[EDIT: When I mention Ren, I specifically mean the one that Kylo Ren will go on to kill, the one that is seen in the comics.]
Alright, I'm gonna call this now while I can. This thought at first came to me as just a "haha what if" but the more I think about it there's not really anything I can think of that contradicts it. The first time chronologically we see Ren is after the current last time we see Cal. Some fans are already speculating Cal could turn to the Dark Side and it's not like he's going to join the Empire after they murdered so many people close to him. And just look at the facial structure, they are very similar and have the same eye color. Cal would be old enough to have gray hair by the time we see Ren's face.
This wouldn't ruin Cal as a character either, since directly linking him to the ST like that makes a much bigger impact than him dying or going into hiding (let's be honest, a lot of modern Star Wars stuff is really just trying to eventually make you like the ST). I think it would also give you a reason to care about the Ren character. I think this elevates both the Cal side and Ren side if this is true. It wouldn't at all make Cal look like a joke as Ren losing to Luke and later Ben is fine, one is literally Luke Skywalker and the other is someone also with Skywalker blood killing a 60-year-old man (Cal being born around 32 BBY and dying in 28 ABY)
They also have similar things they do. Both of them will sneak into places to commit heists. Cal encountering Vader gives Ren a way of knowing about Vader (since he does), and it would explain why Ren is the only member of the Knights of Ren to own a lightsaber.
It's said in the comics that Ren got the title from killing the actual original (or maybe not even the original) Ren, so I don't see it impossible for that Ren to be a boss in the third Jedi game.
Lastly, this might be a reach but the game where Cal debuted and the comic where Ren debuted only released 33 days apart. This means either I am completely wrong (no way they'd make it a retcon as they would surely have narrative plans for Cal) or it was planned from the start (doesn't even need to be a hyper-specific plan, both teams could have just been given a few details to work with and that's why they keep Ren's origins vague).
I only saw one other person make this speculation when I googled it and it was also on Reddit so hopefully it's taken well