I know you've seen a lot of my posts recently, but this is one I really think we should add.
https://www.spigotmc.org/resources/oh-the-dungeons-youll-go.76437/
Generates dungeons, or allows an OP to generate them, based on popular dungeons mods such as DungeonCrawl and Doomlike dungeons. This could enhance the gameplay of vanilla+ servers because it's a really configurable mod that allows you to customize spawn rates, loot, which mobs spawn, etc.
It's not that resource heavy, contrary to popular belief.