The Common Theme
Inexor will end up being kind of a sandbox, and in fact it is already. Users should be able to create their own themes and distribute them among their friends.
However a game needs a bit of a common theme.
The original Sauerbraten theme is similar to that of old-school FPS like DOOM and Quake. Fast heavy metal music, rusty metal textures.
The gameplay of Sauerbraten is unique nowadays. It is fast, like really fast.
My first sense of achievement in Sauerbraten was, that at the second day playing finally the headache was diminishing when moving around!
We do not think a game should be realistic. The reality has stupid constrains, real creativity can begin where we can get over them.
In order to be unique another FPS in gray and black is not what we are aiming for. But the opposite: it should be colorful and weird.
The game should allow us to see things differently, and always discover something new when we exploring the game. Therefore, the base contents should allow building any surreal fantasy.
When being distractive, don't be counterintuitive
Although the theme should bring color into the game and maybe bend the users head sometimes, it should always be obvious what the player actually needs to do. The overall UI will remain a concise and intuitive experience, whereas the maps and gamemodes are allowed to break realistic boundaries with the built-in mechanics.
Inexor's main soundtrack will probably contain really progressive trance or psytrance. But nothing is set in stone, not here and never actually.
The feeling that one should not be too sure about the stuff one sees could become a key element of the theme. Movement of formerly static components like textures, background image could add to that. Even the animated transition between two different map models could be aimed at.