If I could make one retcon to fallout new vegas I would make the legion explicitly pro-ghoul/supermutant.

The NCR being a radically egalitarian yet xenophobic society that hates ghouls and supermutants on a starship troopers level would have made the legion more morally ambiguous and less comically evil than they are now. The legion would still be misogynistic slavers but if they were genuinely the only faction that treated nonhumans as people it would have made picking what faction to side with harder. Mr House would have been the shitty compromise between the two, he may have female sex slave robots and alienate poor people from the strip both human, ghoul, and supermutant alike, but he is more than willing to take everybody’s money. Meanwhile the yesman ending would have been a burn everything down without thinking about the consequences ending like it still is and turning New Vegas into a post apocalyptic Syria. Is the legion still bad even if they are the only faction that is cool with ghouls and supermutants. Yes, they own slaves, but it would also make the NCR bad if they are trying to commit a genocide against ghouls and supermutants. And the NCR needed to look bad in New Vegas, if there was more development time they absolutely would have added more content painting the NCR as not the good guys.

Also if the legion was an ever expanding military force that defeated and assimilated 87 tribes, did the ghouls and supermutants not have tribes? The word tribe is loaded with all kinds of real world colonialist terms but there was ghoul and supermutant tribes and settlements all throughout the fallout games. And the legion being based off rome did assimilate some of the people it did conquer, I’m not saying the romans didn’t treat conquered people like shit, but they had assimilated some of those people into their military force and government. I just see not having ghoul and supermutant legion troops as a missed opportunity, imagine if Lanius was a supermutant, Ceaser’s right hand man, imagine if Caesar called himself the successor of Master, if non-humans were intergrated into the legion in new vegas in the highest and lowest positions of their society it would have made destroying the legion a much tougher moral decision, because not siding with the legion would have been the worst possible outcome for anyone who wasn’t human.

Also there’s some stuff about supermutants being genderless so that would have been very interesting to see how the legion rationalize treating them as people but women not. I don’t think the legion is morally good, I wouldn’t want to change it too much because if they were too sympathetic it would be much like fallout 4 where the moral dilemma was, is slavery bad? But the legion are good villains, and if they had this one good character trait with them, I think it would have added a lot more nuance to the game. Because the choice wouldn’t be slavery or genocide, it would be these factions are evil, picking the third or fourth option somehow makes things worse. Also the only good things you hear about the legion are from Danny Trejo’s character so I just wish they went down the Legion being pro-ghoul route.