Naruto and Ashura were both drop outs and were born without talent, Hashirama was born with the Wood Release and was stronger then most adults.
They're hardly "drop outs" lol. They might have had slow development but they turned out nearly or just as powerful as their rivals. Naruto was born with the Kyuubi inside of him. If that isn't potential, I don't know what is. I will concede that they developed differently, but at the end of the day, Madara/Hashi... Sauce/Naruto... they turned out to grow up very close to each other in power, at least in their natural lives.
Regardless of if someone is marked as a "no good younger brother" or a "drop out", Hashirama had his own talent just like Naruto did, and just like Ashura did. And when Hashirama was at the kage meeting, it was clear that his morals are a parallel to Naruto and Ashura.
The Younger son lived in the elder sons shadow, Madara was born powerful but not only was he weaker than Hashirama he wasn't afraid to admit it.
Both Naruto and Sasuke have gotten near equal screen-time. I'd hardly say they're in either one's "shadow". Unless by shadow you mean Sasuke's darkness. Like I said, they might have developed at different rates but Hashi and Maddy are equal paralells.
Madara actually respected Hashirama.
Sauce and Naruto have mutual respect for each other as well.
The Elder Sons and Younger Sons relationship was vastly different, the only similarity is that Madara betrayed Hashirama.
Really? Can't think of any more similarities? How about the fact that both pairs had an earth-shattering war that ultimately decided the fate of the world? Thats a pretty big one right there, just off the top of my head.
But the same thing happened with the Sannin and the two who did have no realation to the sons as far as we know, Jiraya and Orochimaru.
The Sannin are a
Trio. And that title, sannin, is non celestial. Hanzo gave them that silly nickname. They are irrelevant.
TL;DR: Hashi and Maddy were past reincarnations but when they died, Naruto and Sasuke were both born with that endowment. They are clearly parallels, a common storytelling tactic that Kishi likes to use.