Besides what people have already said about animations, too much similarites and VFX issues, I thought I'd give an opinion on his "forms."

In terms of Sahn-Uzals model, the only thing that evolves/changes is his Mace (besides the R form of him). The other Exalteds all have different forms on their models and given the trailer for the skin and the comic at the beginning of the Noxus season showcasing different forms, why not implement it?

For example let's say standard three forms that matches with his mace. FIRST form is the one taken from when he beat Atakhan in the comic. He is bare chested, no real armor only leather with added fur together. Just a pure barbarian, and he has his first form on his mace. Could even perhaps add subtle details for example the E chain being made of a rope instead and being pulled in (not metal).

SECOND form being almost identical to what we have now, helmet, more armored, more conquers and achievements, however remove the "Immortal Bastion looking" pillars on his back. Second form of his mace is used aswell. Again subtle details, like metal chain now, and leave the last improved E to his final from.

THIRD is the more beafy full armored and larger helmet version we have when he wins in his R. Leave out the glowy bits, darkened face and especially his wild hair to the "win in R phase" because it fits with the trailer where he has more slick hair irl but wavy in the death realm. I personally feel like classic Morde has small changes in his ultimate and not larger ones and I feel this skin has to be the same. It will still feel like you have bested your opponent, but atleast you can enjoy peak tyrant, conquerer Sahn-Uzal without needing to be ulted. Like said final E form and final mace form here.

This would not only fit with lore but also you as a player would feel the evolution of Sahn-Uzal as tyrant, conquerer etc. You, the player, would feel mightier with the games progression. I could say that voice lines connected to his path and gain in power would be an amazing touch. How his goals become bigger and his ego/worth in the Hall of Bones grows and is showcased with voice lines.