The goal is to get to the cheese without a shaman.
Mice have to figure out how to overcome obstacles.
Honestly, I think 4-6 mice is a good amount for the minigame to start.

Obstacles can be
-a simple wall
-moving gears/cogs
-lasers
-death traps
-simple traps
-any mechanism getting in your way to the cheese

Some rounds have Transformation magic.
-bypass sensors that autokill mice
-use scales
-tilt and/or sink platforms
-etc

Various things to help mice achieve cheese
-buttons
-dispensors
-sensors
-scales
-objects
-keys
-etc

Side Objectives
-collect "fraises" for extra points
-save fellow teammates
-"activate" cheese(view examples tab for explanation)

There's many ways for a map to be set up.
So with all that explained, I don't plan for simply "vanilla" mechanism maps.

These examples will explain what is in mind.

1)Mice are given an objective to save an "NPC" held hostage. For fun, let's give this a princess/knight sort of theme.
-Uncrossable drawbridge
-Mice enable drawbridge with some sort of trigger
-Mice enter castle full of traps and must avoid and/or disable them
-Mice come across "dragon npc" and must destroy it by activating triggers
-Mice find door to the princess
-Door is locked
-Mice must find a key and use it on the door
-Princess NPC is activated and thanks you
-Cheese rewarded/activated

2)Transformation magic example. Think of an mouse autokill sensor as a red transparent ground that has no collision.
-Mouse autokill sensor in the middle of the map.
-No clear way to get through
-Mice activate moving platform that goes back and forth across the sensor
-Mice transform while on the platform and get across safely
-However, not every mouse has transforming magic, those that got across have to disable the sensor so the rest can get through safely
-Sensor at end of map that needs 3/4 of living mice to open the next area to get cheese, which is why you should help your teammates

3)-a few number of mice are separated by a "cage"
-free mice need to release their teammates by helping correctly
-below the cage is a large drop
-there are switches/buttons that are labelled
-simply lowering the cage would kill them as the cage would be programmed to lower and disappear, causing them to fall in the abyss
-mice must activate ground first
-mice then activate the cage to lower
-cage disappears, teammates are saved
-etc etc, used to enable trigger at the end to get cheese

There's literally so many ways a map can be set up.