NetHackWiki is primarily a wiki about NetHack. As such, it should contain pages about the game, its source code, its development, and its community. But the NetHack story cannot be told in isolation. Articles about previous versions, variants, forks and patches are also appropriate material, provided they are not confused with information about vanilla NetHack.
Rogue, Hack, NetHack TNG, PatchHack and SLASH'EM are all part of NetHack's family tree. Other roguelikes are not. We should have articles about each, to explain how they relate to NetHack (possibly many articles, if the relations are deep). But we shouldn't have articles like List of potions in Angband. Roguebasin is a better venue for that.
An argument could be made for having some Tolkienish or D&Dish articles, as these are major influences on NetHack.
NetHackWiki is also a community website. Pages about your character, your ascensions, and your (inevitable) deaths are quite alright, but make sure they are marked as such by adding them under your user name space.
Fan fiction, should it ever arise, is also appropriate.