4th keystones?

So Riot mentioned that later down the road, they'd be willing to build on keystones and maybe even have 4 for each tree eventually.

I'm here to ask you guys on some cool concepts for a potential keystone for Precision/Domination/Etc.

I'll start.

An idea for a Inspiration keystone I thought of was something along the lines.

"After selecting this keystone, lock all other rune options and select two keystones from any combination of trees, but lose any opportunity to get minor runes."

I think this would create interesting builds around pumping power into two different keystones.

Just as an example, maybe run both Phase Rush AND Electrocute on Talon, but lose Sudden Impact, Hunter branch, and so on.