Some ideas:
Something like a cavernous level with gold and/or gems on every tile, and a single W guarding it would be easy to implement (and look quite cool), if unoriginal. Ideally the wyrm would be alerted to the PC when (s)he picks anything up - "You hear an angry bellow and thundering footsteps!".
A temple of some sort - there are 5 chaotic temples in the game and no lawful/neutral ones (well, except Ruun's chapel) - perhaps the temples that existed further away from the chaos gate haven't yet fallen to the powers of chaos?
A puzzle, like say a level which is entirely dark ("You go downstairs. It's dark here... really dark. It feels almost as if something is covering your eyes. Your skin crawls") and cannot be lit (when you try: "You hear sorrowful moans! For a second you see hateful faces before the light is absorbed by some strange force!", summons ghosts). There would be a maze - not as hard as the Minotaur maze! - with a reward of some random stuff. It could be the tomb of someone so maybe armour, some weapons, herbs and some bones?
Gate closers: L+ DE Mindcrafter (p20), N= Gnomish Assasin (p20), N= Gnomish Mindcrafter (p23), N= Ratling Ranger (R49)