after the initial breach it seems very boring yeah, even the caverns have more content, what i would do is the following:
-make hell a second world map: from what i gather of fortress mode, caverns and hell are generated as single instances in each embark,i dont know how that works in worldgen but it would be cool if all the underground regions were forming different world layers i mean. there would be a whole map of the underground (caverns) and a whole map for hell itself. the magma sea too.
-make hell as complex as the overworld: the caverns are a world by itself, althought its composed of 3 different types. hell could be as complex and be compossed of different biomes, it could even mirror the overworld. slade would be the along the lowest Zlvls before reaching a second molten layer with no more rock to breach.
-minerals and stuff found only down there. maybe some kind of adamantine-like material for other weapons (adamantine seems good for edged, but how about blunt weapons?)
-evil fauna and flora which thrives in the demonic grounds, some of them lava dependant maybe. breeding and growing them should be a challenge but worth in value
-demon fortresses dotting the landscape, with similar features to the original ones, acting as sites and ruins for the adventurers and the fortress that get to breach into it.
-demons could have different castes as people point, pretty much like angelic beings. with guardians, bestial and FB/titan type ones.
-after breaching hell the player is welcomed by a scouting group, and not by a huge zerg rush, the idea is that the player will have it hard to deal with the welcoming party. but in the case of getting victorious it would later be challenged in time by goblin-scale sieges from the demon fortress.