You know how to make Frozen Heart and IBG builds viable on more champions?

An item that mitigates a flat percentage of incoming damage and directs it to the mana pool, and maybe the percentage can scale based on bonus mana. Of course if you're OOM then there's no mitigation.

It would also serve as a balancing lever for champions like Ryze, because if they want to buy this item (and, yes, it's likely to be mandatory on them) then you can actually drain them OOM over a period of time instead of having them enjoy basically infinite mana.