The other day I mentioned the importance of random tables in generating a dynamic world in which the GM can feel like they are also playing the game.
Well, popping up one my 'reading list' is Konsumterra of Elfmaids & Octopi with "d100 Horrible Dungeon Decor". And some of the entries really are fantastically horrible - in a good way (I guess) - that will really make your 'underworld' a little more than a narrow stone corridor that is handily the dimensions you've assigned to the squared paper that you are using.
This is not quite the sort of random table that produces a 'living world' - that's more the domain of tables designed to be used in play, at the table. But it (along with the vast quantities of other tables that Konsumterra produces) are a great way to kickstart your old, idea-free GM head.