It would be amusing to build an on-hit champ that could crit.
Right now there's the Shiv and trying to figure out how to add to that fun without breaking the game.
You could change the passive to "on-hit magic damage can now critically strike" (this way it won't stack with BotRK and won't stack with itself). You can bring the cost down a bit to compensate. Not much though, even though this only interacts with a few items the fact that it can interact strongly with abilities as well could make it a monster late game item (but honestly in line with something like and IE).
Don't know that we really need a thing like this in the game or that there's a way to do this type of item without breaking the game: Magic DPS on ADCs, a few mages (teemo, fizz, diana), and bruisers. It'd be funny though.