I apologize for my late as hell reply. You're certainly right, Rubick's ability to cast a spell he steals immediately very powerful, and would be over-powered in this game.
Let's say for a moment Rubick himself made it into League, and based off of how League works; Yes, you could do super-mlg plays by stealing a
or
ult, it would probably still scale off of his AD. You are still the one casting it. He would be amazing against mages, yes, but what makes him an incredible support in DOTA 2 would make him dominant here. For example, The ability to have stolen a past
ult, unstoppable force in (with ult),
Flays, you steal that, you use that, you use your Q and done. You would win a team fight off of that. During post-6 laning you not only have your own CC and harass but your enemy support's as well. You may steal things from junglers. Pre-6 laning he still has strong potential because of the duo. His second Q proc gets a lot more use, which is immense, and his W, the stattik shiv-esque ability, just makes great harass. As a midder, sure. You can win lane, build more damage. However, rubick is super squish, and super immobile. Laning with him would be about as easy as laning Nami (as a midder) Sure, you can do it, and sure you benefit from the fast XP, but you wouldn't stand well against most midders. I would also imagine his actual AP ratios would be super low if he WERE in league to compensate with how OP this would be.
TL;DR, Rubick would be an OP support due to his versatility, his lane presence in a duo lane, and his teamfight potential. Sure, he could be good in mid, but there are many other champs who would benefit much more from it. Of course, this is based off of nothing was scrapped from DOTA 2 -> LoL.