I just find that my spell bar gets way to cluttered with each slot being it's own entry. Makes the spell slots feel more like charges than it does actual tiers of power. Spells Should be like the Hex spell, it shows a box after you click the spell to choose what ability to target. I dont think it would be much of a leap to be able to click on a spell and then select the level you want from a similar drop down/pop up
Had a similar thought and think this sounds like the simplest as well as effective UI. Having the same spell multiple times in the hotbar but without any visual indication of which one is the higher level spell means you have to hover over the spell, wait for the pop up (I know it's relatively quick, but doing that a hundred times over adds) and then determine if that's the level you want to cast at.
