You have to kill specific zombies that specifically drop heads, and then throw the heads onto all the lapis lazuli blocks (they're blocked off by a fence or something) The zombies continue to spawn even when you have enough heads though

To find a good idea for a map, just find something nobody has done before!
You could make a really dense forest, a maze, etc.

so you want original ideas for an arena fight? How about an arena that consists of multiple rooms, each with their own elements to it (not neccesarily elements like fire or water, just parts or special things). There is an option somewhere to close the doors between these rooms, but if you close the doors you have to kill an entire wave of the mobs that spawn in that room before they open again.

Apart from that you can move between rooms and mobs will follow you. Each room gives you a buff and/or a debuff every time you clear a wave in a room, a chest spawns in a specific place that contains a random piece of loot related to the theme of the room (if you have a nether room, a piece of fire protection armor or a fire resist potion or something).

once you leave a room, it will stop spawning new waves. Once you enter a room, a full wave will begin to spawn, and the next wave will spawn after a certain countdown.

Players can set the amount of waves they want to conquer or go survival mode so they keep going until they die, and the amount of waves they get.

Include some sort of scoring system for both modes.