Here's why the Kass ult change and any compensation won't lead in a healthy direction:

The first thought that a lot of people had after Kass's ult got nerfed extremely hard was that Riot would buff other parts of his kit and that this would hopefully make him more healthy, or they were happy that he was gutted. One or the other.

"They can buff his Q/E base damages back and give him back his silence!"

"More damage!"

Basically the idea is they're gonna give him more damage.

The problem is that his ult is now too short-ranged to be used aggressively, recklessly even since I used it haphazardly before the nerf. Now why is this a bad thing? Surely it's not a bad thing that a Kassadin player cannot be reckless and aggressive with ulting. Because most of his damage is going to be from range now. While he won't be able to jump over any wall or flash ult past your front line to kill you, any compensation to his basic abilities will still make him extremely frustrating since his damage will be so high from range and the punishment for a sloppy ult will all but disappear since you don't need to rely on it for damage. Even if the damage is low, it's very important to using Kassadin well, and even if it's hardly a skillshot, there will be times when even the best will miscalculate their range or just straight up get juked (has happened to me sadly) and this made him a bit easier to deal with in niche cases while also making him have more risk associated with killing somebody. In the one game that I played Kassadin after the changes, I spent the majority of teamfights playing as a ranged champion. I'd QE and then use my ult to get into range for more QE, or if we were brutally ahead during that particular fight, I'd RQWE to kill somebody. You're cutting down his options for healthier (his ult is always going to be problematic no matter what) gameplay, and even if the relative risk for using his ult is questionable, moreso with the rework reducing the CD on it which is a buff to AP Kassadin in skirmishes, relying on landing his ult to do damage is the healthiest gameplay you can hope for with this champion.

Also, if you do compensate him, although his W is arguably the most broken ability in his kit aside from his ult, that's probably the next healthiest place to buff him.

I'm even willing to go as far as saying that aside from the nerfing of his numbers and the Q change to no silence, he is a more toxic champion than before the rework. His reliance on his ult before made it so that he had to land a pseudo-skillshot, and while it was too forgiving in cost sometimes, the general idea that "if you miss R you're fucked and your CDs are longer so Zhonya's might not save you" was a lot healthier than "RQWE hit Zhonya's then your ult will be guaranteed to be up by the time Zhonya's ends." While it was a pretty good idea to lower his CDs and his burst, his ult CD was lowered to the point where he has very little risk to going in unless you have a trigger finger with your CC.

tl;dr Kass's ult needs to be powerful and take up a lot of power budget so that he has to land it and use it extremely offensively to prevent him from devolving into a skirmisher that can hit you hard from a short but relatively safe range and then goes in to finish you off. He should be forced to go into melee, and this ult change is removing the option to do so. Any compensation would be problematic