An idea for a dwarf fortress Elf variant mod is a Deep Elf mod, or a Drow mod. With a possible friendly faction in the caverns (Maybe the second or third). Instead of asking your fort to "Not cut down a single tree", they instead ask you to replant the same number of mushroom trees you cut down. In exchange, they give you some good materials from the underground, like lost relics they discover, caged cave beasts, and if you earn their favor, maybe a special metal (Maybe a metal that mixes plants and another metal. letting whatever it's forged into repair themselves when you put it near plants of the caverns). As for what happens if you make them angry, I think a more terrifying way would be to let their raids spawn with one of them riding a forgotten beast into battle (Not one that will automatically kill the rider, but still a forgotten beast). They could also show up with other underground dwellers for assistance. Maybe, if you want more danger, you could have them have a tomb that let's acts like the necromancer tomb, but instead, it can summon the misty forms of demons in exchange for a downed body and a casting time. Place this in their settlements or something (and if a settlement is accidentally found on a cavern level, their settlements would let you still come down, but also let you either slowly integrate that settlement if you respect it's people, pay some stuff, and have good relationship with whatever draw group owns that settlement).

A variant of goblins would be ones closer to folklore dragons. Effectively wandering nomads who double as wandering merchants. Offering good food, some artifacts, fire, and maybe some special materials. You may even be able to ask these travelers to get you specific items or resources (With a present chance to succeed, of course). Maybe there can even be a variant of this group who specialize in mercenary work, and you or a group of your dwarves could go out and pay them to help attack something, or help protect your settlements. (These groups would likely not exclusively be goblins, but have some other people around them.) Maybe these groups will even have certain affiliations with existing groups. Like being hostile or friendly with certain nations, making their actions more or less cheap, or keeping the fortress from certain actions if they're hostile to the fortress's nation is hostile.

A variant of dwarves would be tricky to make would be dwarves. for good reasons. Deep dwarves aren't really that special in my opinion, way too close to the normal dwarf. So why a variant of dwarf like people? They don't have any settlements; they are a scattered people. Often known for their creation of strange items. Random artifacts with innate magic. With their masterworks rumored to take on stranger abilities. Not always positive, mind you. They can be any person, as long as they have the [inchanter] tag. This would have fortresses try and recruit people they otherwise wouldn't bother.