Erwin Smith, Commander of the Survey Corps., died at the hands of the Beast Titan in season three of Attack on Titan. He almost survived by becoming the next Colossal Titan, but Levi Ackerman eventually chose to save Armin's life instead. Looking back on that choice, one wonders how season four would pan out if Erwin Smith survived to find out what was hidden in the basement.
The obvious change would be that Erwin would remain commander of the Survey Corps, which would allow Hange Zoe to remain as a squad leader, much to their relief. This would probably be the turning point concerning the Rumbling considering how Erwin Smith approaches his enemies.
Erwin Smith's Death
While working with the fact that the future actually is set in stone in Attack on Titan, Eren would probably be even more reluctant to go through with it than he was when he initially found out about it. When Eren returned to Paradis Island after demolishing Liberio, he grabs Hange's collar through the bars of his cell and demand to know what to do. He actively demanded a different option, and Hange could not give one to him because they hadn't come up with a way around Marley since finding out about them four years ago. Later in season four, Hange laments over not coming up with a plan and felt guilty for pushing Eren into what he thought was the only solution.
While Hange isn't a bad leader in the slightest, Erwin takes the cake for the best of the best. Erwin could probably be just as shocked as the rest of them to find out the humanity existed outside the walls, but he already had hope that it did in the first place because of his father. Erwin would have used that reveal to solidify his own beliefs and would already start working on a plan to deal with the already existing war with Marley. He would have come up with some idea eventually since we've seen how fast his mind works when coming up with a plan to capture the Female Titan.
One of Erwin's greatest frustrations about his survival would be the fact that he was now a titan shifter. Shifters make good foot soldiers but not great commanders. The Beast Titan can at least talk in his titan form, but Erwin would need to hang back in battle, so he could assess the situation and mete out orders. Putting one of the heaviest hitters on the back burner doesn't make the most strategic sense, and even if Erwin can finish a battle in one go, it's not the most productive move. Erwin would probably not use the Colossal regularly, much to his frustration.
This leads to the most important question to date: would Erwin Smith support the Rumbling? To quote Floch, Erwin was a good leader because he was a demon who was willing to do anything to succeed. As long as a soldier knew that, and was willing to follow those orders, there would be no doubt of victory. Floch has stated that one of the few reasons he was following Eren as a Jaegerist was because Eren was another demon. While Erwin might not support the Rumbling wholeheartedly, he would be able to recognize how easy a victory it would be if there weren't any other competitors. This is the same commander who recognized the best way to capture Annie Leonhart was to corner her in the bustling populace was Wall Sina, which he was formally reprimanded for by his superiors. Erwin told them with certainty though that the ends justified the means, and his other military leaders reluctantly agreed.
The Commander
Erwin Smith would not agree with Eren's Rumbling. In fact, Erwin would be furious that Eren walked happily into the obvious bait laid by Willy Tybur and the Marleyian government during their war declaration, but he would absolutely use the Rumbling to further his own agenda. He would ignorantly follow Eren's plan to have him killed after enough countries on the mainland had been wiped off the map. Erwin would easily allow Eren to get his hands dirty for the rest of them in order to keep his home alive and safe for at least a little while longer.
Erwin Smith's survival would not change the resolution of Attack on Titan, but it would serve as an explanation for Eren's actions without the needed private conversations he had with his friends after he was defeated. Erwin would figure out how to use Eren's insanity to secure the safety of Paradis Island. As Floch had believed, Erwin's ruthlessness and willingness to get blood on his hands would advocate for the freedom of Paradis.