Interesting Kassadin Rift walk

I just realized an interesting idea as i looked at 's splash art.

I'll get right to explaining what I would personally like to see happen to his ultimate if it were in the process of a complete re-work:

R: Riftwalk

• Passive: While on cooldown, Kassadin gains 20% CDR and 5/7.5/10% increased Ability power (the CDR gained from this passive does not affect the cooldown of Kassadin's Riftwalk

• Active: after a 2 second channel, Kassadin leaves the physical plane, unable to be targeted but being unable to attack for 3/4/5 seconds. During this duration, Kassadin can not be seen (even by pink wards), and his movement speed increases by 30/45/60 for the duration. When returning to the physical realm, Kassadin generates an explosion (250 unit radius around him) dealing 200/280/360 (+30/35/40% AP) damage.

There would also need to be a global sound that alerts both teams that Kassadin has used his ultimate.

• Cooldown: 75/65/55 seconds

Riftwalk does not currently do Kassadin's character much justice as he simply warps around and doesn't have anything particularly spectacular to look at. He is depicted as being able to generate explosions from abilities like his Null sphere in the lore, but I don't get that feeling when playing him as of right now. (the ability is also quite broken)

This ability would bring Kassadin back to life in terms of being an assassin and being very mobile, but not simply being able to warp away from situations on a whim (unless ofc you are low on mana).