This is a gamemode where you can do whatever you want, to start playing first click "create new game" then a new game will be created and saved on your computer, there will be "Load" button that with it you can load a saved world, you can invite other players, in this gamemode you can do whatever you want like: changing the max level, add new tanks, add new polygons, change the max level of a stat, add AI enemy and AI helpers with different abilities... If this gamemode is added the game will be a lot more creative, and a lot of people who wanted to test their ideas can test it. (its now can be an upgrade to sandbox mode)
Note for the creators of diep.io: If this gamemode where added the game would have a lot more hours played bicouse it would be hard to be bored, and a lot of suggester/thinkers would play a lot more.
Advertising this idea would make a lot of suggesters/thinkers to play with their dreams.