I also want Karkat to become a little relevant. Imo, I think the reason Karkat isn't ever allowed to be "relevant" in canon is because of his aspect specifically. Blood is an aspect of connection and relationships, and because the two timelines are split apart with different needs, Karkat acts accordingly to what the people in that timeline need the most or whatever. In the Meat!Timeline, he's just a guy with a boyfriend going on another space jaunt, and he has nothing to do with the actual plot, he's kind of just there for emotional support or something lol. And this is because everyone in Meat is doing something about their problems, they're actively working to better themselves and are already thinking about what's really wrong with them and the way their lives were going.

In Candy, Karkat became a whole rebellion leader; he's super badass! But the Troll Rebellion is just something that happens that has no real "impact" on the larger Canon of the story. It has nothing to do with Dirk or Calliope or the Helltier thing. And I think this is kind of intentional, because throughout all of the Candy arc and the Epilogues, Karkat and Kanaya are the only two characters who have retained any of their senses, as in they don't make stupid, dumbass "Out Of Character" decisions throughout the whole thing.

They are also the only characters who aren't gods and never attain godhood during the game. I think when Echidna wanted to go and take a look at them, she knew intrinsically that they would be the two characters that would be able to hold everyone in both timelines together long enough to see things through to the end. And Karkat also is one of the characters who straight up tells John that even if the world was completely fake that it doesn't matter to him, because he, unlike the rest of them, is stuck living this life. And he has to make the most of it, which is why I think he's so divorced from Canon in a way. The others get to choose what lives they want to lead without the story holding them back, but they refuse to change where as Karkat can't. He doesn't have plotpoint meta bullshit powers, he can't go back in time and make things different, he can't see into the future to predict what the next best action would be, he can't move the planets to get what he wants etc. He's just a soldier with a sickle, and like a really desperate race of people who are willing to follow him into battle.

I guess another way to say it is that Karkat is essentially one of those characters that the narrative can't actually do anything about or for, because he's stuck there and has already changed and grown while the others haven't. Karkat's gotten over a lot of the problems that held him back when he was younger, like his blood color, and whether or not he even fits into any quadrants. Because the rest of the characters haven't changed yet, there's nothing for him to do narratively, I guess. There's no character Arc because he's the one who changed the most by the end of their adventure and who just grew up and got over all of those problems on his own. After all, most of what he went through as a kid was him having to learn so many lessons about getting over stuff.

I saw a funny thread a while ago when the Vriska helltier upd8 happened, where someone theorized that Karkat will be the next person to go through it, but that he'll only be in there for like 5 minutes. And then when he comes out, the others will wonder why he was in there for such a short amount of time and why he doesn't look any different, and it'll be because he has already changed for the better. We also see Karkat get over a lot of his past relationships during the Epilogues. He hates Gamzee more than anyone and not in 'that' way, he walks away from the Jade and Dave situation on his own after getting sick and tired of them going to bat for Jane, Kanaya and him are the only people who point out how everything is weird and strange and how none of the humans besides John seem to have noticed but when nobody listens they both decide to just fight back anyways against the growing tide of fascist Jane bullshit lol. He even became a better fighter! And someone who can actually hold his own in a battle without having to have Kanaya or one of his friends knock him out before he does something foolish (Like run headfirst into a giant Lord English mouth laser blast.)

All this to say, I totally agree and I do hope he becomes relevant or whatever just because I think he's also really cool and if they just squander all of the potential he has by idk offscreening the results of the troll rebellion or just making it end in some stupid way that doesn't result in Karkat helping and doing something for once, I'm going to be a little disappointed </3