Well I guess I haven't really done any super "original" servers because I can't think of anything that hasn't been done before. I had an idea where you have to go through an obstacle course to get to the goal at the end and whoever gets there first wins, but that's not really original - it's just a deathrun without people controlling the traps.
I've also done servers using the hide and seek gamemode which few people seem to use now, but I've only gotten like 3 people joined at once.