Modiverse provides many ways to modify the sandbox game, you can add props, create NPC's, Weapons, vehicles and even very intelligent actors capable of interacting with players... However, you can only go so far before you need to have a higher control of the game play. In sandbox you cannot control the flow of the game, there are no rounds, no objectives and no clear way to play.
If you want to control all aspects of Modiverse and built a game where players engage in a flow (rounds, objectives, tasks, etc.) then you'll need to create a Modiverse game-mode.
Prior to reading the various articles under the Game modes section I recommend you read the Proxies Primer article to understand how Modiverse works through proxies; effectively insulating you from complex UE4 process and allowing to to implement just your game-play.