I'd like a tank item that lowers all (except true and pure, obviously) damage, but also reduces your damage output as well. You'd become more of a wall, but at a cost. And, well, tanks without damage and enough crowd control tend to get ignored if they're not a threat, so it wouldn't be great on every tank especially when behind and almost useless on most bruisers/juggernauts.
On the flipside, I'd like a double-edged sword item in game, that makes you squishier but deal more damage, preferably for mobile AD assassins. Make the reward easier, yet riskier. Hypothetically. I could honestly be full of shit for all I know.