Several demon lords didn’t make it into 5E, and now that I’ve looked into them I see why. 5E’s demon lords aren’t necessarily fun or interesting, but they’re all at least somewhat active. They have goals, usually some form of domination or omnicide, and they actively work toward them. The lords who didn’t make the cut were much quieter, content to sit back and enact small-scale plans unless a group of heroes stumbled into their path. It’s logical, and it increases the verisimilitude of the setting, but that’s not what the designers wanted. They wanted monsters PCs could punch, and somebody who avoids being punched has no place here.
Here are brief treatments of some demon lords we lost. You can use them in your campaign as background noise or targets of lore. If you want a more nuanced enemy than the standard demon lords, you can use one or more of these instead with only a few changes. There’s a lot of material about them if you look for it, enough to make any one of them a proper campaign villain.
Kostchtchie: A paragon frost giant. Kostchtchie is an archetypal berserker, so enthralled by combat that he occasionally forgets he has magic and minions too. He is friendless and expansionist, and he wants to spread his reach far and wide, but he is limited in that the only followers he wants are frost giants. He may begrudgingly accept certain barbarian worshippers if they meet his exacting standards for savagery, but he rejects everybody else outright. This gives him a startlingly limited footprint, and in an edition where frost giants have their own specific god he just didn’t have a place. If it suits your campaign, you can substitute him for Yeenoghu.
Pazuzu: Everybody’s second-best friend. As a birdlike creature Pazuzu rules the skies of the Abyss, and other demons lords allow it because there’s nothing they want in the skies anyway. He’s almost as old as Dagon, and he’s survived several wars mostly by being a small enough threat that nobody wants the hassle of fighting him. He has no ambition, and he would rather corrupt a few pure, innocent people on the Material Plane than rule over any land he’d then have to defend. His trademark trick is appearing any time somebody says his name three times in succession, promising help that inevitably leads into dependency and vice. You can use him instead of Fraz’Urb-luu if you’d rather your demon lord lie in wait instead of actively campaigning for followers.
Dagon: Awake friendly Cthulhu. Dagon is an aquatic demon lord, as old as time itself, and he shares his extensive knowledge with those who follow him. He has a begrudging alliance with Demogorgon but he rarely associates with anybody else. He has no patience for anything that lives on the land, and only the most monstrous of aquatic creatures follow him. For now he’s not interested in expanding his domain or flooding all worlds in an infinite ocean. He’s content to lord over his own pocket of the Abyss, sharing secrets with his worshippers and letting them carry out his will. He doesn’t match directly to any existing demon lord (except Pale Night), but you can use him instead of Orcus if you’d rather work with krakens and kuo-toa than undead.
Malcanthet: The token sexy lady. Malcanthet is all about hedonism and satisfying one’s desires. There’s almost nothing about her that doesn’t deal with sex in some way. Aside from the obvious problems with this concept, hedonism is now the domain of Graz’zt, who used to just be “that demon lord who was imprisoned by a human for a while and got Stockholm Syndrome.” If you really think Graz’zt should be female you can use Malcanthet instead. If you want to use both, they have a legendary rivalry in which each insists the other is a spurned lover, and their war can be the seed for an entire campaign.
Sess’Innek: Lord of reptiles. Sess’Innek’s portfolio occupies a strange place between ambition and protectionism. He wants reptilian creatures, like lizard folk and yuan-ti, to rise up and form empires that will last for ages, and he’s willing to aid them in their fights against mammals. But he doesn’t care about Abyssal politics at all and he’s not confident enough to test his might against demon lords, so he’s placed himself in relative exile. Other demon lords don’t care about him, nor do the gods and goddesses of reptilian races, because he’s unlikely to ever be much of a threat. He’s a good candidate to swap for Baphomet for a different sort of patient brutality.
Lolth: Mistress of drow and spiders. Lolth is in 5E, but she’s been promoted to a goddess. If you’d rather pull her back to a demon lord, you can, but there’s no strong need. She’s mostly here for the sake of completeness.
Obox-Ob: The deposed king. Obox-Ob is the former ruler of and most powerful creature in the Abyss, but he died in a critical battle. One of his aspects survived and grew in secret, and now he wants his title back. He isn’t mindlessly savage, but his rage controls him rather than the other way around. He’s mostly biding his time, destroying everything within reach out of spite, waiting until one of his cults comes up with some world-rending plot so he can push them the rest of the way. You can swap him with Demogorgon, but for maximum impact do it in the middle of a campaign and let the players deal with the fallout.