I will say that one of my absolute favorite little flavor pieces in nethack and SLASH'EM was the elf-orc and angel-demon hatred. It was a nice feature that these two groups would rather fight each other than you and that they would fight to the death even if they were aligned to the player!
I think Crawl could benefit from a similar system, at least between angels-demons where they fight each other on the spot. To me at least it feels like it breaks the narrative a bit when Mara and Mennas put their differences aside and team up against the player. Unless you are undead, daemonic (or a follower of Beogh!
), Mennas should prioritize demons (and orcs) over the player. This is part of the reason why I wasn't happy with the decision to remove pacification of holy monsters dependent on piety with one of the good gods. It didn't come up often, or in any meaningful way, but it added a level of flavor to the game when a devout worshiper of, say Zin was not attacked on sight by angels in service to Zin and TSO.
Sometimes its easy to forget that the Flavor text really does add a lot to this game and makes it more engaging to explore. HO of Beogh is one of my favorite combos, not because its particularly good past the mid game, not because the interface starts to tediously deteriorate with orc allies swarming around covering up the floor tiles so you can't see where anything is. Rather I enjoy the class so much for all of the flavor text. The orcs have personality, they say things like "character, you're my only friend" or "don't worry character, it is an honor to die for Beogh", or even "I don't want to die". In a similar vein, there was a hell portal vault that really resonated with me, it consisted of a hallway with dwarf skeletons, plate armor and weapons strewn about- leading to a portal to hell. For a minute it felt as though I was walking through the ruins of Moria, the dorfs had dug too deep! People notice things like that, and its part of what makes exploring DCSS such a rewarding experience. I also enjoy that monsters and unique will trash talk your character.
Having the enemies interact with each other makes them more real and sheds light on their own wars and allegiances outside of the player. I personally would welcome back good gods pacifying angelic beings at high piety levels. I would support orcs and elves attacking each other on sight. In fact, I think it would be an amazing experience for a new player exploring to come across an orc and an elf band engaged in a heated battle. It also opens the door for some really cool altar vaults, like a Yred-TSO vault with a Profane Servitor and an Angel locked in a death battle. Crawl has amazing mechanics, and I do enjoy them, but its also important to add depth to the rich history of the dungeon and its inhabitants.