My opinion:
First off, Sion is the most fun champion in the game right now for me. Then comes talon. Sions rework was pure genius. Old sion was kinda fun in a way but theres just no comparing. Old sion was yet another generic AA meatball who took a quarter of a brain cell to operate.
Aatrox should be more elegant and skill based as his lore describes. Allowing the player to determine the outcome of a fight with reaction times and out-plays. His "inspire allies" part has always been a bit of a misinterpretation to the player base as far as im concerned. I don't think it was meant to describe literal buffs for allies. It was the way his kit was designed to pounce in mid-fight when his team is potentially losing to completely turn the tides.
Aatrox should inspire your team to continue fighting just from the very fact that he's there at the right time. Again, this doesn't have to mean literal buffs to the team. His arrival should mean a shift in the team fight. The problem with sions old r is it was too damn strong lol. Sion got reworked but people just want aatrox now to turn into old sion and i HIGHLY doubt riot is going to re-use an old kit skill to fit a newer champion after discontinuing it.
Sions old ult was bland and insanely strong for the lack of skill required to use it.
Riot will eventually take a closer look at aatrox (in the distant future T_T) and hopefully break away from his mishmash of roles and the most prominent of those roles is the one we don't really need more of, the AD attack speed fighter (zZzZzZzZ). Why bother playing a new champion if he's going to feel just like those 20 other generic aa fighters? Not saying some attack speed isnt healthy; Vi is a good example. But if you want to play old sion or some rockem sockem champion there are sooooo many available.

Its also a personal distaste I have for these types especially since there are so many. Aatrox just barely feels like a more interesting variation. I feel we need more fun / interesting kit designs which feel as though you have more control like
. Shes the only REAL bruiser swordsman in this game. Oh and
who is awesome but I just feel like his kit lacks skill-cap depth. Ekko is awesome but he's clearly a mage assassin with no actualy swordplay aside from his auto's :P
Aatrox was so close. I mean it felt like they were on the right track and at one point they just said "Fekit. Throw some attack speed in there and send him out."
TL;DR I don't think recycling an old ult that was purposely phased out should be some copy paste fix for Aatrox.