New Champion Concept - "Shape Shifter"

Working on a champion concept and could use some help, so far I've only got the ultimate and passive down.

So the point of this champion will be a shape shifter. A champion that can fill many roles due to their abilities and can be built many ways as well, focusing on adaptability and utility but also able to drop the dps if need be.

I'm not decided on a name yet so let's just go with "Shape Shifter"

Passive - Selling an item returns its full cost, rather than a percentage like with every other champion. This will synergize well with his ultimate. Can be nerfed to scale with level or drop to a flat 85-90% instead. Seeing some abuse cases here where he could just build 6 early game items and sell them all later without sacrificing anything. Edit: Dorans items and gold gen items already sell back for only 50%, so maybe keep that rule with Shape Shifter, or raise it slightly for him only to 60-70%.

Ultimate - Now this shit is gonna be truly game changing if played right, but at the same time has plenty of counterplay. So essentially, "Shape shifter"s ultimate will have a 180/150/120 second cooldown at levels 1, 2, and 3. Once activated, for ten seconds you can steal an enemy champions ultimate and use it for yourself. Rank of the ultimate determines the rank of enemy ultimate used. (Example: If you're level 10 and enemy is level 11, you will still only get to use the enemy ultimate as if it were at rank 1) Range would be comparable to a Leona ultimate. Shape shifter, being a shape shifter, would change into a spirit-like version of the enemy champion while using the ultimate (Think Yorick Revenant in appearance) so as to not have to make 130+ animations for this champion for each different ultimate he could steal. However for balance the ability will have two drawbacks;

1. Shape shifter can only steal an ability that is off cooldown, thus planning ahead will be necessary and there's nothing stopping the enemy from simply casting the spell before you can steal it.
2. Shape shifter will cast the ability based off his stats instead of the enemy. (In other words, if you steal Lux ultimate but have 0 ap you're only going to be doing base damage, with no way to proc her passive either because he wouldn't be stealing the passive just the ultimate)

I'm seeing ridiculous counterplay against champions like Ekko, Tryndamere, Kayle, Zilean, etc who rely on their ultimates to survive or keep someone else alive more than 1 second in a fight, or champions like 1200ap Veigar who you could literally one shot with his own ultimate, to the point where you won't even want to play those champs against him. Meanwhile, champs like Illaoi's ultimate will be useless to him other than the base damage because while it would spawn tentacles, he'd have no w to proc the tentacles with. Still looking for a way to balance this. If he ults a champion whose ultimate is charge based, he gets full stacks (Example, corki ultimate would give him 7 rockets) and if he ults a champion whose ultimate works off base abilities alone (Karma, Heimerdinger) he will be able to use their skills for the purpose of the ultimate alone. Exceptions include Nidalee and Orianna, I have no idea how to make their ultimates work. Maybe make their ultimates unavailable for steal.

That's all I have so far, could use some help on other abilities. Think something a shape shifter would want in a kit. Adaptable, flexible, but not able to 1v1 someone who was designed for the way you're playing him. Example, even going full ad he shouldn't be able to 1v1 Graves unless he can successfully outplay Graves and use his own ultimate against him. Would love to see some hybrid ratios to really complement how he can fit into any comp, whether he goes ad, ap, tank, or a mix of the three. I can't really see him being able to be balanced with more than 300 range either so keep that in mind.