I think Aurelion Sol's design is one of the purest in the game, in that he's almost completely based around a single mechanic (his spheres), yet that mechanic is so rich in gameplay that it opens up a ton of possibilities. However, I also think he's held back by a lot of design conventions that alter his gameplay in ways that are not really necessary. His kit doesn't really need to be gated by mana, since he's short-ranged and fragile, but he still uses mana because mana is the default resource in League. He doesn't really need to deal increased damage with his spheres when using W, which should just be a range adjustment tool, but he has that effect because it makes the ability worth picking. His Q damage is so low relative to the power of his kit that it could just deal no damage, and only stun. He's a champion who could legitimately be okay with having only one single source of kit damage, yet he still has his power budget shifted onto conventional power he doesn't really need, which has also held him back from having a much more interesting ultimate.
Personally, if I were to change Aurelion Sol, I'd take out some of that extraneous power and focus on a) making his passive smoother to handle, and b) giving him an ultimate that fits him better. Given how difficult it can be to kite with him already, removing Comet of Legend's passive and instead giving him much better base movement speed would also make his maneuvering a lot less janky.
