I've got a few I'd like to suggest:
- Anti-aliasing (AA) and anisotropic filtering (AF) options. Whether that's MSAA or post-processing like FXAA, it's always nice to have this sort of thing in-game rather than having to force it from the graphics control panel. Same goes for v-sync as well, although it's less of a deal for me because Grimrock probably isn't a super-fast game.\
- Window options. Some games don't allow you to toggle between fullscreen, windowed and borderless window, and in my opinion that's a pretty big crime. Players should have a choice as to how to run the game, and as a heavy multi-tasker it really sucks when I have to use hacks and work-arounds to get a game running in a window.
- Alt+Tab support. Similarly, games that don't allow Alt+Tab are annoying, to say the least. I'd be very happy if Grimrock allowed for smooth Alt+Tabbing.
- Sound options with individual volume sliders (not just toggles). Music, ambiance, sound effects, footsteps, master, whatever's relevant - there should be a way to change game volume outside of Windows' mixer. I use headphones that are quite sensitive, and even with my computer set to low volume, some games are too loud, so it's nice to be able to change all those individually as needed from within the game.
- Colourblind option. I'm not colourblind, and I don't know how much of Grimrock depends on colour, but it'd be great to see this if it would help the game's usability.
- Subtitles. Yes, I know the game probably has no dialogue. However, a lot of dungeon crawlers use the sounds monsters make to give the player valuable feedback, like whether there's a secret room nearby, or simply what direction you're about to get jumped by. For the hearing impaired, having some sort of subtitle or even just an indicator when monsters are near would go a long way towards making the game accessible.
- Full keybind support with multiple keys per function (i.e. move with both WASD and arrow keys). A lot of people like to switch between configurations (for me it's for comfort reasons), and it'd be great simply to avoid having to rebind keys. I'd also love to see support for thumb buttons on mice, if possible. Once again, this stuff is also important for gamers with physical disabilities.
- Multi-monitor and 3D? Okay, not really essential but I still think it'd be cool in a game like this. :p