That PROJECT cinematic looks so cool, but lets be honest. Zed should have wrecked them all.
My thoughts so far for the "they are coming" is this. A light and a dark side. Light side dealing with healing/support, dark side with damage/carry, I think its gonna be a mage main.
I think it's pretty similar to the image I attached in the spoiler. Basically, the ult will control which form (light/dark) you are in, and then the abilities change accordingly (like nidalee).
Alternatively It could be two of those face thingys that float around together, then depending on which ability you use, a particular face performs that task. Then the ult would be both of them at the same time, possibly they power up a basic ability or they combine and explode.
Just my thoughts