to be honest, for the splitscreen mode, you could change it to any image such as a jungle or a desert since it is 2d. However, for single player I'm not really sure since it's 3d and you can't really just upload a picture to the background. As for area suggestions, I would say maybe a jungle or forest, a desert or a facility at night with search lights moving around.
The thing is that, since each user map has a completely different theme (except for the more generic ones), choosing e.g. a city background would be unsuitable for a jungle themed map. Thus, it needs to be a more generic background.
I could allow each player to upload their own backgrounds for their maps, but this would create inconsistent aesthetics throughout the game (and you can't trust the players to always upload good quality images).
I will put some thought into what would make a better background. Until deciding, I find it best to play safe and keep the generic skybox. Please endure it :)
Maybe you could add a certain option in the map maker in which the players choose a background from a selection of backgrounds that you placed in the game?
Edit: I definitely still enjoy this game without backgrounds, this is only a request to make it even more enjoyable