As a Vi and Kayn two-trick-pony, let me suggest some ideas:
Shadow kayn has a fun and unique assassination style: Zoom in through a wall, drop a champ, maybe lower another one to half-health, and then book it the hell out of dodge. Problem is, he doesnt do more damage than Rhaast because conqueror exists, and he lost the one rune that gave him the immediate up-front burst he needed (old Dark Harvest), and the movespeed on his E is great but with mobi boots and ghostblade it feels irrelevant. Some small damage tweaks would be nice, and to compensate maybe reduce his E speed so squishies don't get nuked without reaction time (even with wards)
Rhaast also has a fun and unique style of winning fights: just keep fucking battering them. Kled-style. Don't let up, or you die. If you get out of the fight, you're at risk of dying. The best way to deal with him is to grievous wounds him, and burn him down. He feels a little overtuned with damage, and this is entirely at the fault of Conqueror tbh, i don't believe he needs significant nerfs unless conq keeps being too good on him (and doesn't get nerfed)
now for Vi, my favorite. She feels kind of like a bootleg tankier version of Camille right now, but with a little less skill expression (people see Camille as a high skill, and Vi as a medium/low skill. Tbh they're almost the same but thats just my opinion from playing both.) Currently she's getting some possible changes on PBE but it's been like 4 cycles and i don't see em anywhere but PBE still, so i'll suggest some here
Okay, so she's just worse but significantly tankier Camille, that's fine, because Camille is a true damage tank-buster and duelist, with assassin elements and a single-target+collateral knockback movement denial
and Vi is an.... armor cracking tank buster and duelist, with assassin elements and a single target+collateral knockback(and damage)
Hmm. Let's see here
Her Q is an integral part of her kit, the charge-up punch is what makes Vi into... well, Vi. It's dodgable (unless you flash-q and catch people off guard), it's telegraphed, and it's her primary and only source of CC outside of her ultimate. Keep it as-is.
Her W is important because it enables her to punch up the tanky bois and win, as well as giving her a jungle clear. Perhaps adding an active to it, like Varus's W, would be cool, but not something i have an idea of.
Her E is incredibly useful, but is basically just an auto attack reset with some small AoE, it's basically just there to make Trinity Force a good choice for her. I don't know how to change that significantly without breaking her power budget
Her R is what removes skill from her kit, because it's a large damage point-and-click inescapable chase. I love her R to bits, it's super fun, but i think the damage needs a tune-down, it's a catcher spell instead of a burst spell, in my eyes. Catching is what it's good at, and it'll almost always be followed by an auto, q, or e so it doesn't really need the sizable damage.