Vriska's ultimate feels really weak. You'll be lucky to draw 2 and deal 4 damage with it for 8 loyalty, which doesn't feel worth a 4-turn charge time. Assuming you can stockpile her "next time you roll" chances by using that 8 times in a row after accumulating loyalty, that adds another 12 turns of charge time to get her ultimate at full power. That's a pretty ludicrous time investment compared to pretty much any other planeswalker ultimate of similar potency.
Jade might want to add player's choice of +1/+1 counters or -1/-1 counters to things, since as written she can't shrink things past their base stats. (-1/-1 counters and +1/+1 counters annihilate each other when placed on the same target, so it's pretty much the same functionality as far as counter removal and bookkeeping goes.)
Tavros doesn't feel black to me. Virtually everything his card does is white - add loyalty, put weenies in play, and exile stuff. I'd put him in white/green. Also, I feel like he should be able to take control of creatures instead of putting fiduspawn in play - canonically, I don't recall him using a fiduspawn at all during the session. I know controlling creatures is usually blue's style, but limit it to, say, attacking and blocking creatures, and it fits green/white pretty well. (Yeah, I could suggest the same for Vriska, but her card has a good focus on her luck manipulation aspect, and her abilities all have good synergy with each other. Tavros really doesn't.)
Equius doesn't feel red to me. He turns into a big creature and fights (green, although red
does do some fighting) and makes creature tokens. (Also green.) Maybe let him break artifacts as an ability to deal damage to an opponent. (Artifact destruction is a thing red does, a thing Equius does, and has synergy with his robot-manufacturing ability.)
Eridan looks good, but his first ability feels kind of bad for a five-mana planeswalker. Nice loyalty gain, but one white mana?
Karkat, Meulin, Feferi, probably a few others: Probably want to be reworded to add "You may", so you're not forced to give bonuses to your opponent's creatures if you don't have any. Also, Fef feels bad if your opponent isn't playing a red/black deck.
Speaking of Karkat: Dayum! Dude has a one-turn charge before he Mind Twists your opponent into the ground, and can do it every 2-3 turns afterwards.
Caliborn will kill his own player before ever becoming Lord English if his alternate play condition is used, assuming a standard non-EDH game with no lifegain or use of Cronus to autotransform him. That may or may not be a feature.
Petty fixes:
Gamzee: Spelling error on indestruct
ible.
Kankri: Ultimate should read gain 10
life.
Jane: Enchantments from her ultimate don't need "as an instant" in their activation cost - that's default for activated abilities. It's when an ability is sorcery speed that that needs to be specified.
Architect: Ability should read "
devotion to a color"
Dirk: His ultimate might want to read "Put target permanent in play into it's owner's graveyard." That should bypass indestructible and regeneration, and reads a little cleaner.