Mana Runes Reforged

I just noticed, I don't think any runes related to mana regeneration have been revealed yet. It would be interesting to see a keystone or normal rune that offers unique mana regeneration or something along those lines, like gaining mana by dealing damage but spells cost more flat mana (meaning missing spells is really bad but hitting spells is really good, for mana). Could be an inspiration keystone. It would open up new options for certain champions, like letting them skip a mana item for a defensive item, or something like that, if they play right.