Aatrox is really good early game but starts to fall off mid game. Part of the reason he falls off is because of his ult. His ult does not have very much benefits.
His ultimate ability is called World Ender, but it doesn't feel like the name. The current stats are nice but it needs more. However you may think that it would be a very overloaded ultimate, but it has a fairly long cooldown, and singed has a very overloaded ult.
Maybe he could keep half the movespeed boost after the initial boost is over. Also he needs bonus range and attack speed like his old ult. In addition, his passive E ability healing should be doubled. Also it would be nice if his rage refreshed while in combat so you don't die while in a teamfight.