Talk:VPBE/@comment-24795478-20150227133522/@comment-1330314-20150228071154

What about:

Kassadin partially phases into the Void and gains bonus movement speed. While Riftwalking, he is untargetable and ignores all collision, including terrain collision, but performing any action other than movement will phase him back into reality. While Riftwalking, Kassadin drains his own mana and builds up stacks of Residual Void Energy, increasing the mana drain. Kassadin can spend stacks of Residual Void energy on his other abilities to empower them. There is a 7/5/3 cooldown to the toggle.

So basically what you're suggesting, with the terrain-hopping part intact.