So... what is it about Kassadin that makes him such a balance nightmare?

The combination of mobility + save (you cannot miss it) burst. He has pretty much the same patterns as LB, but she´s more focused on single-targets and has to land a hard to hit (and blockable) skillshot in her e for her full combo, which somewhat balances her out.

They just would have to decide to either switch his patterns to sustained melee-dmg + mobility or to less save and more single-target focused burst and he´d be fine, they just really fucked up their first rwork-attempt by keeping all his patterns the same (and therefor ending up gutting him completely).

Short CD's

Super strong blink.

Burst damage.

They attempt to balance him with an abysmal laning phase.

it's the combination off insane mobillity and point n click burst on range. He cam apply 2 off his damage skills from 1350range with out a chance to miss. This makes him an insane cleanup as u will eat his burst befoe(or exactly)the moment he comes in vision range(1100). Combined with the slow on E, this makes also his ganks/followup on ganks really massive, as he will sow u from a up to 1400 distance with no counterplay. At the same time, his massive mobillity makes it nearly impossible to gank him, especially with silence removed from LB and Talon.

SO in order to balance hi kit, he should get more (melee)dps focused and less bursty, and his cc should either get counterplay or also short range. I think shifting more damage to the passive on-hit on W would be great, and changing E into a skillshot, a bit more narrow arc than now but sligthly more range and travel time, so it would be not reliable on higher range and allows counterplay. Besides that, he realy should get his silence back, maybe not on the point and click Q, but somewhere in his kit. The poor guy still uses it in his joke...

in the wise words of dunkey, "This is kass, his ult's flash" but really though he is way to mobile

The same reason will be super strong unless his numbers are unsatisfying. He is way too mobile. Lucian is less mobile than kass but has significantly better range. Long story short is utility is highly undervalued by designers because damage is more noticeable.

It's the ult. Great mobility, and short CD. In my opinion, the best way to balance it would be similar to what they did to Lee Sin's Safeguard, and make the ult have a longer CD and potentially a higher mana cost if it doesn't hit any enemy champions. This makes his easy escape and chase a bit more limited, while maintaining his ability to engage and burst.