How would Tsunade treat Sasuke if she learnt the truth about the Uchiha Massacre?
There is a lot to unpack there.
Tsunade's views about the Uchiha take after her grandfather's ideology. To her, the Uchiha name denotes trusted allies.
Growing up, Uchiha Kagame was a figure she would often encounter. My forever ago interaction with @blurredsharingan's Kagame led to my headcanons that the man was the first encounter young Tsunade had with the Uchiha clan and his friendly approach towards a toddler Senju princess made Tsunade very fond of the nin.
This fondness led to her being friendly with all Uchiha's she encountered as she grew up. They were commands. Friends. Men and women who had her back out in the battle field. People she experienced the horrors of war with. People she bed for and bled with.
Any clan related jibes were always only for fun and amusement - with the sentiment of light heartedly mocking the animosity their ancestors had clung onto.
Then came the fact that on Uchiha Fugaku was on the same gennin team as her brother (HC shared with @fire-hearted-father-blog's Fugaku). Fugaku had the natural talent to make the Senju princess want to match his constant deadpan. Following Nawaki's death, their interaction was limited but each time they crossed paths, Tsunade's sole focus was to annoy the stoic Uchiha. She got a ridiculous amount of amusement out of her attempts. They clashed a lot but Tsunade had a lot of respect for the man and his talent.
The news of the massacre had hurt her - she'd lost friends. Allies. People she had grown up with. It had shaken her a lot. Until her return to Konoha, she'd not faced the reality of what had happened. It was a tragedy of horrendous magnitude.
If/when she finds out about the planned Uchina coup - she wouldn't have been surprised. She'd never understood, nor accepted the discrimination the Uchiha's had faced. She regrets not having done anything to change that. Another regret fates bestowed upon her.
How she would treat Sasuke - as the last of the Uchiha's, she would understand some of his feelings as she herself is the last of an old shinobi blood line. Officially she would have treated her as any other nin. Outside of her official role, she would have tried to show him the respect she had for his lineage. I believe if Orochimaru hadn't fucked things up, she would perhaps had a chance to instill in the Uchiha the same loyalty many of his clan members had towards the idea of Konoha.
I believe there was the possibility of a tentative friendship there.
Bonus: One of my favorite HC's shared between me and my main Sasuke is that Tsunade and Saskue end up forming the most bizarre friendship following the war. It shouldn't have worked with all that has happened and the generational gap. Most of it is snark and jabs shared between the two with the seediest bars as the background but each encounter just so happens to be something both look forward to. He always seeks her out for advice - the fool. She tries to play the wise sage (pfffft) and gives him the best advise she possibly can but its always shit and yet the idiot trusts her (WHY?) to go through with it and always end up in trouble. Still he always comes back for more.
