since inspiration is about rulebending and cheating gamerules.
what if there was a rune, that allowed champions without mana to utilize mana from items as a resource?
https://i.imgur.com/KjqDNuT.png
they would gain an actual mana bar (with items as their only source) and mana costs on their abilities when equipping the rune. (would probably be quite the balancing act to assign every champion without mana reasonable costs to the individual abilities, but could be worth the effort).
the mechanic would function similar to
rage bar where they can still cast abilities when theyre out of mana. but if they have the mana to cover an abilities cost, it gets empowered in some way. like, say 10% increased damage.
technically this could work for mana champions too, just by increasing their base mana cost by like 50% or something for the extra effect. theyd have to be more careful though, since they could actually run out of mana.

working for non mana champs..
doesn't waste the mana when she buys
.
and
around.
or
if mana wasnt a wasted stat on them.
or
more easily on garen. etc.
or