I think that the Madara and Sasuke had a great potential of interaction that was not exploited. The scene where Sasuke comes to attack Madara and get stabbed, paired with a brief flashback of Hashirama saying how Sasuke looks just like Izuna and giving him something to stop Madara is one that had left me confused for a while, and I've seen others express similar feelings.
It looks incomplete, I read some argue that it almost looked like a part of the scene was cut out. I don't know what happened, or if this is true, but either way, it's an opportunity to speculate.
At first, since both Hashirama and Tobirama insisted that Sasuke looks so much like Izuna (and they do look a lot alike), I thought what was missing was a stronger reaction from Madara, grief or hesitation, something that could show it was difficult for him to hurt someone who has the same face as Izuna's.
But then I thought, but he was either blind or had the rinnegan when he 'saw' Sasuke, perhaps is it just that he couldn't witness the resemblance, maybe the rinnegan sight makes it harder to see detailed features of one's face and Sasuke's chakra is so jarringly different, he isn't fooled.
That's when I remembered something else. Izuna's design is very much made to resemble Sasuke's, but they don't look exactly the same (the lip line Izuna has but Sasuke hasn't is the main reason why I am saying this, why include it if it's not relevant). We know Madara can somehow tell clones apart from the original, so, perhaps, he just saw that Sasuke didn't look that much like Izuna.
With this in mind, imagine this scene, but instead of thinking Madara sees a boy with Izuna's face, he sees a pale imitation of his little brother, close enough for Hashirama to think they look just the same, but not enough for him to not tell the difference.
I wished Madara's reaction would have been stronger in this scene, but not in the form of grief anymore, but as anger. Because Hashirama tried to fool him with Izuna, tried to beat him in such an underhanded way, no matter how far he has fallen, this tactic is cruel from Hashirama and it would sting. Because this kid thought he could replace Izuna, because they all thought his love for his brother is shallow enough that a lookalike can fix him.
I really like the idea of Madara being a walking paradox as well, here, I made him monologue about how useless it was to try to use his memory of Izuna because he knows it isn't him, but also argues that he is dead, that he only has a purpose to fulfil and that his humanity is out of reach, all the while he is visibly pissed at both Hashirama and Sasuke for very personal reason.
He is insane, he wants to put everyone in an eternal dream while always refusing anything that isn't genuine or true.
