Mod Ideas!

So, my mod called Expedition X is a Vanilla+ overhaul of normal Minecraft, adding a new progression system that makes you take your time and not rush through a world, plus it adds a bunch of really cool structures, mobs, biomes, dimensions, blocks, and items! It's a pretty beefy mod and I was wondering if anybody had any ideas for me to potentially add! Anything is welcome as long as it fits the theming! If I like your idea enough I might add it! (credit optional)