No - anything you like theme-wise. The only restriction here is that it needs to be a parser game.
The main requirement is that is needs to be text-based to a large degree. Graphics are OK but play needs to be controlled primarily by text input, parsing, text output. If your game fits, i.e. it's not just an arcade game, then that's fine. It sounds like it'd need to go in the 'freestyle' category though, for anything not immediately recognisable as a classic parser game (see info on main page).