This far I have planned the user interface which is going to have a nice style (I guess). When thinking of the theme I was first planning to place the game in a mansion and then on an island with a lighthouse. Boring.
Now it's second day and I felt that I need something different. Then I had an idea: a large spaceship with cargo container halls as central place. They can be just like regular caves in fantasy roguelikes. Places are connected with elevators. It could be cool to create a 3D level structure so elevator shafts (and everything else too) would be placed in their real location.
I've only started day 2 so it's time to plan stuff. The structure of the spaceship and levels, what items you can find, how about some vending machines? And I need some kind of story, what's happening in the spaceship and what the player needs to do.