you start with an idea i made the boogiman minigame for xbox and pc in my minigame the boogieman wears a pumpkin on their head and try to kill the humans its so when a human kills the boogieman they dont get a point but they get to be the boogieman and if the boogieman kills an human he gets a point the humans get unlimted lives until they kill the boogieman and the boogieman gets one life and when the boogieman kills like 4 his points are four and then he dies and he keeps that score and he kills that rounds boogiman he does not get to add a point and the humans wear all iron and the boogieman wears diamond and the pumpkin on their head its pretty simple game maybe that will give you an idea on how to make a minigame.and there are four rounds and the person with the most points at the end of round four wins.
You could use PocketMine and make your own plugin with php. sekjun9878 has multiple tutorials on this-http://www.minecraftforum.net/topic/1790333-new-tut-official-pocketmine-mp-plugin-programming-tutorials-by-sekjun9878-chapter-6-first-plugin-with-snippets-released/