May I direct you to the modding forums? There's a fine assortment of mods to choose from, ranging from those that crank up the difficulty (Fortress Defense) to those that flesh out the world (Genesis) to those that are just fun (mega-goblins) to those that are close to a total conversion (Wastelanders, etc).
Just browse around and give one a shot. If you know DF pretty well, picking up and playing a modded version is easy to do.
Tried the mods. Played Genesis till I got tired of werewolves sieging me at every turn. Played Corrosion till the infected came riding giant jaguars (really broke it for me...). Played various other, but currently decided on Legend of Forlorn Realms. Anyhow, survival is still not really a problem even when LoFR has enemies with metals just short of cotton candy, and yet the routines at the moment is destroy sieges, clean up, and kill more sieges. Not too hard when defenses are set up correctly.