it depends on what kind of skills you have for developing a game.
If you're a coder, you can do a player base editor ; it'll make many users happy.
If you're a 2D artist, there is some stuff you could do.
If you're a very skilled 3D artist, as well.
If you can create good animation, it could be interesting as well.
It's better to contact me here for this kind of thing: https://www.managames.com/mail_en/
Note : I got this kind of offer regularly, but so far, only one 3D artist was actually serious enough to go through with that.