If there's ever going to be one again, I'd suggest Biome Bundle , it's an amazing custom biome mod, with lots of custom and randomly generated biomes, rocks, structures and trees! And the best part is, that it's completly vanilla, so the server won't need to have any sort of plugins in order to work. Only downside of this is that the mod is currently outdated, and can only be played up to 1.7.10 currently. But anyway, it would be amazing to have such beautiful landscape, while playing online vanilla Minecraft!