I'm going to fire off a bunch of ideas here... A creeper/ghast/blaze temple A giant villager trading station Mega farm Giant lapis/gold/iron/emerald/diamond pyramid A giant cactus temple (nice lobby... holes in the roof for cacti to fall down... a giant auto cacti farm above... ) Casino! Giant redstone calculator Cookie generator
a giant adventure map of any theme! (space maybe?) could have multiple events/contests! (cold make a living off entry fees)
How about a mall? It costs a little to set up and stock at first, but when its done people will come running. Or maybe some kind of prison place, made with stuff like stone bricks and iron bars.