I play Rubick quite a bit and the problem I see with him is that his ability to support before level six is pretty bad. Telekinesis isn't terrible but it has a long cool down for helping in lane and fade bolt is just a nuke with a minor debuff that can also push in the lane and mess with CS for your core. But other than that, he doesn't have much. No amazing movespeed or attack dmg.
So I was thinking what if Rubick was reworked in a way similar to invoker where he has no real ultimate and can use spell steal early on. However, what if the stolen spell is much weaker at lower levels of spell steal so he isn't just nuking people for a 1/3 of their health at level 2 with fade bolt and another spell.
Also, I was thinking of lowering spell cool down and making aghs allow him to use teammate spells but not ultimates and stolen teammate spells are weaker and have a lower duration(on the fence about this one however)
I am open to discussion