Customization Pixel Art
Seeing as this is the 8-Bit MMO and it's all about players creating the world I assumed that you would get the ability to create your own NPCS and Monsters.
Have it that maybe players can purchase customizable mob spawners, then the player can name, and do the pixel art for the monster, maybe there would be different levels of mob spawners costing more but having higher difficulty. Of course you would have the problem of People making like penis monsters but you could always have a flag function, and if enough people flag it an admin could check if it needed to come down...
I wouldn't think this would be to hard, I mean the Monsters don't have moving sprites so you wouldn't have to worry about that...
Jamie Gordon commented
This: http://www.realmofthemadgod.com/draw.html is a really good art maker it could maybe be based off of.