Eh. If they made his R's stack duration down to 10s or made it scale with CDR, he might actually be playable. His damage already requires him to be in melee range because a lot of his power has been loaded into his W much like Gragas's power was loaded into his W during his own rework. Both were safe melee champions that had good poke and could kill at range, both nerfed because they were melee killing at range. Gragas is still playable as AP and will still one-shot any squishy with a W+E+Q combo when you have a Lichbane and another big AP item. Kassadin has more mobility, but doesn't have the CC Gragas does nor does he have the damage Gragas does. His mobility is gimped because short CD dashes are considered too strong, though it's okay if it's on a relatively short CD if there's CDR, like Gragas's E with CDR or Lucian/Graves/Riven/Nidalee/Ez if he lands a Q or two/Kha'Zix (Who also gets resets if evolved)... There's lots who can jump very often without punishment and some do it over notably longer ranges, like Zix who has a base CD of 12s on his max rank E, going down to 7.2s with maxed CDR, Lucian's is based on his passive chaining but sometimes can be completely ignored CD-wise with the right ability/AA combos, Graves just has to AA to reduce his own CD and with 40% CDR, it goes from a 14s CD to a 8.4s CD...
All of this and Kass's R is a 2s Flash, but its mana cost doubles each time its used within the last 15s. Changing the CD to 15s or making the 15s be reduced by CDR could definitely put Kassadin back into the game without overloading him since his combo ratio still only totals, at max ranks for all, 610 BASE damage with 2.0x AP ratio and 2% of his max mana, which without mana items is 2% of about 1600, which is 32. With RoA and Seraph's, it's 2% of 3400, which is still only 68 damage. The mana ratio is near useless. Even with stacks, at max, it only does 6% of his mana which would cost a fortune of your mana (800) to do about 204 damage. Mana items on him are only to make his R spammable. Make the CD lower and he can build mana regen and CDR instead and he won't be OP, but at the same time, he also won't be so bad.