I think custom skins/textures would be great in the game.

Diep.io is a multiplayer action game available for web browsers, Android, and iOS, created by Brazilian developer Matheus Valadares. Players control tanks and earn points by destroying shapes and killing other players in a 2D arena.

The smash hit game! Control your cell and eat other players to grow larger! Play with millions of players around the world and try to become the biggest cell of all!