I don't really see that as a bad thing tbh that Rod of Ages is bought by both mages as AP tanks.
That's because it's not cost effective at the moment you buy it, it's an investment with it's drawbacks.
So I wouldn't particularly mind if a scaling item that's not cost effective but an investment for tanks is implemented into the game, if bruisers want to buy it, it should be fine as long as the item's balanced.
So instead of negative stats (which I doubt that Riot would ever consider, otherwise there likely would've been runes with combined negative and positive stats already) I'd rather see items being an investment (cost inefficient) for later on (becoming cost efficient) and balanced around that.
EDIT: How about an item that gives (maximum health / 100) % of your maximum health as a shield as an active for 15 seconds with 40 magic resist and 40 armor? :)
Indirectly it has a negative stat to it for bruisers.
It'd also be a counter to percent health damage and Guardian Angel would usually be better unless you play against a team with 2 BotRK's and one with Liandry's Torment.