I'm sure we all know the biggest argument against this concept is the fact that Rubick's spells would be easily distinguishable from the opponents' in their perspective, which is even a major part of Rubick's lore : in which his claim to fame was his ability to perfectly mimick the opponents' spells and deceive them into thinking they casted them on eachother. There's also a statement for this mod which is the fact that a hero-unique spell effect will allow Rubick's allies to destinguish which is theirs' or the opponents' (ex. the ults of Dark Seer & Shadow Shaman).

My proposal for this situation would be to use this effect in the same way the game already handles illusions (ex. Phantom Lancer) and invisibility (Invis, Techies mines), make it so that Rubick's team sees the stolen spells as green or with other custom effects, while their opponents will see them like normal spells. It wouldn't work as a mere cosmetic since it can actually affect gameplay unlike items, so if anything it will have to be built into the game by default (also because custom particles aren't allowed on the workshop).