Hear me out: Ranked Aram
A few changes would need to be put in place, of which I would suggest: A ban phase, 3 champions per team, no one can roll these champions. 2 or maybe 3 rerolls for everyone, every time. This would decrease the randomness that is inherent in it, but still force you to be more flexible than normals. Need to own 5 or 6 champions for each role at a minimum to play ranked. 30 or 36 champion minimum (5 assassins, 5 fighters...etc) No more weekly free champion rotation. (Looking at you, Ivern.)
(Also, I wouldn't mind the ability to sell champions back to the store, so you have a specific roster that you can adjust, if you want.)