This idea is loosely based on a comment on the big nasties in hell: they're supposed to be undefeatable (mostly), but if you manage to beat them, you can keep beating them and it simply gets routine and easy. Plus, you have nothing else left to dig towards.
Here's my idea: the first layer of hell works like it does now. You have slade all over the place, flaming pits of doom-and-garbage-disposal, and all the familiar goodies/baddies you've come to expect and love of DF's unique flavour of hell.
But... not all of it is slade. There are holes and cracks, that a skilled dwarf can dig through. Can you guess where they lead? That's right! Worse hell! So... The creatures get automatically generated, like before, but some of their bodily materials get copied from the originals and tweaked to be slightly different. Maybe demons made of copper that's 10% lighter and 5% weaker than normal copper, or hellhounds with firey muscles that are unnaturally tough and strong. The materials surrounding this hell would also be slightly different versions of the stuff we all know. If you're lucky, you get bright-green adamantine-but-not that's heavier but just as strong, making it perfect for warhammers. If you're unlucky, you get a slightly different-looking microcline. There's a few off these altered materials scattered all over the place. Mods could also introduce materials that only start occuring past the n'th layer of hell, just like some things only occur below the n'th layer of rocks or the n'th layer of caverns.
This is, of course, assuming that it's even possible to generate creature types on the fly. The forgotten beasts lead me to believe it is. It also assumes that additional z-levels can be added to the bottom of a map. I don't think this is in yet, though it'd be fun to mess with. Doing the same up into the sky would be awesome, at least, but that's another topic.
Next layers of hell would simply follow the same rules, getting another bit scarier and weirder, and probably more dangerous. Dig down far enough and you'd probably encounter bubblemonsters that can't even be popped anymore!-... that is, if you survive the fluffy wamblers from HELLLLL..... layer 81 or so.
The idea behind this all is, of course, that fortmode players always have a next, simple challenge to follow rather than having to make up their own. It'd be fun for the more gamey-style players. It probably wouldn't bother the other types at all, and it'd probably bring them something new to mess with too. If you're feeling suicidal and one "dug too deep" isn't enough for you, you just keep on digging until the world warps around you, the monsters are eldritch and the beer starts tasting funny. And that's, me laddy, where the fun begins!