See this is what I love about Speculation
You got your ideas, I got mine. You do you, I do me
But I don't know, Personally it would just feel kind of wrong if Sephiroth's Trailer came in the timeline before Hero's Trailer. It's like this
Hero's Trailer made it seem like Hero came out of nowhere. He's here to save the day from Dharkon's Army. he was getting bodied, but thanks to the other Heroes, they were able to free the other Dharkon clones
But you go to Sephiroth's Trailer, the same Hero (Erdrick/Arusu) that was the legendary reveal in the 2nd act of Hero's Trailer, is out there getting mopped by Galeem, and is about to get erased, before Sephiroth Slashes Galeem like Butter. He's also destroying everyone until Cloud does damage to him, and even then he turns into Safer Sephiroth in the end
This way you got the stakes going higher. It's pretty much the Shonen Anime Power creep build that those show uses
But if you reverse it the other way, It's like the opposite of the Power creep, it's like a Downgrade in power. Sephiroth just killed Galeem, destroyed everyone but Cloud, and even when Cloud did a number on him, he goes to his Final Form, meanwhile Dharkon is nowehere to be seen
Sephiroth gets warn off somehow, Dharkon then now decides to show his Hand, and among all the fighters, All the Heroes and Link seem to be the only ones left? Not Cloud or anything like that. That's why I think that order is weird, and it raises alot of questions to be honest, like how were only Hero and Link able to be the survivors, why was Hero so far back when saving link, and other stuff like that