Diversity is good, However try to refrain from adding too many entities, as there are only 4 seasons you can have them show up in, and if you get too many friendly races, and hostile races you run out of times when they can come/show up. You can control that with their TRIGGER values, but that only makes them show up at a certain threshold.
I'd recommend looking into your current races. And seeing if you can multi culture them (what does that mean?) by using Castes you can make say your Existing "Dunemen" also include "Desert Dwarves" and even "Sand Elves (exiles from normal elves for crimes of touching metal? Vanilla Elves seem to exile based on Oath Breaking, Lying, Assault, Slavery, and Treason)"
For these "rogue"/"nomad" type entities it works great as it's just a band of exiles, and ruffians that just don't agree with their homelands's rules making their own, crude civilization. If anything I believe they are one of your hostile civs (Dunemen) they could be upped to mid to end game as with inclusion (if you make them multi culture) of dwarves would allow them Steel processing. Through control of SKILL_RATEs you can make it so invasion forces would have squads composed of those that would use the desired weapons IE humans can use bows and crossbows, Elves bows, and dwarves crossbows by making their learn rates for those weapons faster or impossible. Makes for interesting seiges when you know a hammer squad is mostly dwarves, and a spear squad would be elves, with a mix of archer squads.
Icedwellers sound like they should be Nordic in nature, resembling Vikings (with Berserker castes that favor long swords and battle axes, with enragement combined with a self interaction (or pass it on interaction) to give them a temporary blood lust boost)
Hellspawn and Corrupt dwarves seem like they can be combined much like Dunemen, Desert Dwarves, and Sand Elves?
If Hellspawn and the C-Dwarves you go the route of multi cultured (castes) Succubi/Incubus Slaver/Masters to herald and manage the Hellspawn(human) and Corrupt Dwarves thralls. Since you're planning on nerfing the Hellspawn, caste them out more as well, like "hallow" castes that are gaunt almost mummified skeletal humans which are numerous cannon fodder, to fresh cursed humans that just suffer from discolored skin and eyes, to favored cursed humans that were given gifts from their dark masters. I'd exclude Elves as they have their mirror Dark Elves
Since most terms Hob is Hearth you could include Hobgoblins as a smaller weaker/faster caste of Goblin, and include Orcs as the larger caste of goblins. Making the vanilla goblins more diverse (Hobgoblin > Goblins > Orcs)
I've been meaning to get back into modding DF.
Some ideas since I've only ranted about Massive multi caste-ing your current races:
MEGA-BEAST Shadow Lord: A Shadow Lord is rumored to be a source of boogeymen, or a ruler of them; the relationship hasn't been solidified other than they both comprise of the same material that crumbles when severed. Basically Shadow made material manifest a Shadow Lord has the unique ability to change form periodically (healing all injuries) and releasing shadow goblets that can form into fledgling boogeymen after each transformation. Even though their form is ever changing, shadowy eyes-like orbs seem to remain constant and damaging them provokes a panic'd reaction. (main core is UPPERBODY and LOWERBODY but there are 3 severable/pulpable HEAD parts that also double as THOUGHT and are of shadow FUNCTIONAL tissue so that damaging or removing all 3 kills the shadow lord, that is if you do so in time. Adventurers will probably make mince meat of them (players op) but AI and world gen should have a blast.)
SEMI-MEGA Wendigo
https://en.wikipedia.org/wiki/WendigoRandom Diseases (Health and Mental): have dwarves (and other races) have a self interaction (permanent) that can give them (low chance for each one) conditions such as asthma, bi-polar, depression, etc making fortresses randomly have dwarves that get out of breath easy due to their condition, or just suffer from random spurts of negative (or positive) emotions, or blindess/deafness, or they just can't use their legs, etc.