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.
I see where you're coming from, but imo sometimes it's okay to have a comically evil group. Fascism is comically evil in real life after all, and doesn't look kindly on (what they would see as) "diseased" individuals like mutants and ghouls.
Besides, even though the Legion is comically evil, some naive chud players still side with them. So even the most comically evil group isn't totally unpalatable to idiots, just like real life fascism!
Counterpoint, if the majority of ghouls and mutants were siding with the explicitly bad slaver guys then the player would feel a lot less bad about killing them, and unless you specifically went out of your way to not kill them, you get an ending slide that says that you were responsible for starting a pogrom against mutants. There’s people who played fallout one and wanted to side with the master, I think the legion is the closest faction there is to that in new Vegas because there already is some bits of lore about them being somewhat tolerant to ghouls, and the ncr being the opposite.