Hashirama makes WAY more sense to be an antagonist than Madara. And I'm prepared to die on that hill
See, cool as Madara was, there were 2 things that kind of don't make sense to give your "final" antagonist
Firstly, he's kind of inconsistent. He wants peace so he's going to trap everyone in an eternal dream state of utter bliss to eliminate war. Ok, that tracks. But BEFORE that he's going to kill...pretty much as many people as he can. And not just right before the battle, he's going to ensure his apprentice creates the bloodiest regime in shinobi history and actively incite as many conflicts as he can. Like I get that they'll be "alive" in the IT, but it's a bit of a hollow victory
The second thing is, Madara was kinda right? About everything. Rallying with the Senju DID lead to the Uchiha's downfall. The distribution of the tailed beasts led to A LOT of death and suffering. And the shinobi world would only be at peace as long as they had a common enemy
Hashirama meanwhile was wrong about literally everything and his dreams got spat on the MOMENT he died. His best friend believed he couldn'ttrust him, and he was right to. His brother created some of the most fucked up was crimes imaginable. The clan he worked so hard to ally with was betrayed by his brother's most trusted students. His plan to build peace by establishing MAD went south immediately because literally no one cared about releasing demons into poulated villages. The village that he specifically founded on the basis that children shouldn't become murderers, not only makes children murderers at the age of twelve, not only lowers that age if they have enough potential, not only has a disposable army made out of indoctrinated orphans, but they have a YEARLY. CHILD. EXCLUSIVE. DEATH. TOURNAMENT. And not even for money or family or shit it's literally so the kage can wave their dicks around
Imagine Hashirama a decade after his reign started, and he saw all the atrocities committed because of his actions. All the wars, the execution of innocents, sheer devastation he unintentionallycaused. And He. Is. Miserable. Not even mad, just disappointed, literally all he wanted was to spread love and peace, and they can't even accept that. So hes leave. No final battle, no peaceful death in bed, just a retreat into the natural world you love so much.But then one day, a descendant of his best friend falls into his home, and he's kind, and hopeful, and he wants the hat so he can make your village better. So you fix him up, and train him, and send him back to his friends. And he almost immediately comes back telling you the saddest story you've ever heard. How a little girl was turned into a bomb(yet another of your fuck ups), how the rival he'd just started to befriend(Who started ninja-ing at 5!) was used as her suicide tool, how their sensei was nowhere to be found
And you just can't take it anymore. He WOULD find a way to make peace. No matter HOW many eggs he cracks. Even if he has to destroy the village that was HIS dream. Even if he has to face the embodiments of all his greatest mistakes. The very last of his best friend's clan, and not only a jinchuriki, but the last of his wife's clan. It doesn't matter. They'll thank him soon enough