Well, as childish as it may seems, Scratch 2 is a easy programming language using blocks to code. As far as I am aware, you cannot have multiplayer. Its a 2d engine that relies on pixelart. I have some experience with it.
I have a simple story that might also fit the bill;
Science has gone mad etc. resulting in (humanoid?)monsters that can only reproduce by unbirthing human fetuses, resulting in them hunting pregnant women. There is a human team that is genetically modified so that they can kill the monsters, retrieve the fetus and unbirth it, ending in the fetus being cured. You can only retrieve a fetus by killing the carrier first.
Both sides have a impenetrable belly. The monsters have a thick bone-like tissue in the skin around the belly that stops bullets whilst the humans have a carbon sheet surrounding the belly that also stops bullets [you cannot see the protection on/in the belly on both sides, its just to avoid killing the fetus in a gunfight].
Gameplay wise you have 2 teams; the defending monster/human team is pregnant while the attacking monster/human team is not. The attacking team must unbirth all of the fetuses while the defending team must A: hold out untill the timer runs out or B: kill the attackers. There are no respawns in this mode.
Or you can have a mode where a team must escort a heavily pregnant VIP while the other must unbirth the fetuses or kill the VIP. The escorting team wins when the VIP reaches its goal. Both sides can always respawn.
There are 3 classes on both sides; a scout [fast, can only carry 1 fetus and has a small amount of health], normal [normal speed, can carry 2 fetuses and has a normal amount of health] and a heavy [can carry 3 fetuses and has a large amount of health].
The amount of fetuses you carry positively affects the health regeneration and negatively affects your mobility (jump height, movement speed and stamina).Post too long. Click here to view the full text.