Riot, since you are going to be removing Atma's

Yah know.. That's not a terrible idea, as that type of passive would be a bit more justifiable on an item that actually gives health, and Sunfire Cape is a tanky item with some extra sustained damage. It could also work as an alternative to its current scaling using damage per level as it would increase as you build items.

The main issue I'm seeing might be the damage potential for those building heavily on health, but that may just mean the numbers would need to be tweaked. Perhaps using bonus health rather than base health could be another option as well.

The entire reason they're removing Atma's is because the health -> damage was a bad idea for an item. Rewarding efficient defensive itemization with offensive potential doesn't work unless a champion's kit is designed around it.