Personally I would either nerf the range or make thr cooldown on his teleport. I think his other abilities are just fine the way they are its his ult that bothers me. So how would you nerf kass and please be realistic when you answer. "I would delete him from the game" or "I would nerf him to the ground" are not acceptable. I'm talking about realistic and fair nerfs to kass.

Take the damage from his blink down or away completely, but also reduce the mana cost to compensate.

Make his silence only last for .5 seconds.

Increase some the wall sizes so that he can't blink over all of them, or just code them that way.
Increase the amount of stacks to get his slow by 1.
Reduce his silence length.
Decrease his base numbers.

Not all of those combined - those are just ideas.

The constraint Riot is going to be looking at is "nerf kassadin without destroying SR viability'.

When you have this constraint, solutions are a lot harder.

So essentially we have 2 options to look at - 1) Create an item that counters him. 2) Rework his kit so he's not as op in dominion, while retaining strength in SR

Recode his ult as a dash instead of a blink.

This fixes his ability to jump every wall in the game, and doesn't affect him on SR that much since most walls there aren't very thick.

Ult gains charge system; 1 charge every 30 seconds, max of 3 charges stored at once.

E is now a knockback Syndra E style, range slightly reduced

Swap passive and W (as in, passive is now autoattacks deal flat bonus magic damage and recover a little flat mana, w is now passive small magic block + the AS bonus, active to increase that number temporarily)

???

profit

I like that Ult charge system. Neat idea.

Honestly, I think the biggest thing about his kit that needs nerfing is his Ult. They can't make it into a dash because it would be completely against his lore and identity in every way (he's a rift walker - he leaves the current plain and returns elsewhere). But, that's definitely the most annoying part about him in Dominion.

Health relics give less % mana/health (for him specifically) while also granting vision of him to the opposing team for some duration