I'm always a bit confused when you speak of races. There are creatures and there are entities, and I never know which one you mean with race.
Procedually generated creatures are just that, creatures, not a civ/entity. But if you can do this, it would be a great improvement, because that would be the first time that dfhack (or modders in general) have done anything to mod procedually generated creatures. Making FBs and Titans better is awesome, and if they can be targeted with scripts, I wouldnt even mind giving them a metal-skin layer or a material-force-multiplier that protects them. Because atm most of them are pushovers.
I wont do much with multiple subraces, I just thought it would suck if too many diggers come all the time, so rather have a rare, single digger approaching you, instead of every unit in a big army. Thats where the idea with the skulking race (like kobolds) comes from. First you get attacked by goblins... then you get attacked by goblins and maybe a lone goblin will come and open a path. I can even make him look like a miner/sapper and give him appropriate names and professions. Fits a lot better then just making any creature dig. I think it will work out great.
Megabeasts and Frost Giants and Titans could approach from the surface and deconstruct walls, while some sneaky miners come and dig into your fort, together with FBs and one digging race from the caverns.