-Create some more reasonable non-gimicky goals that you maybe still haven't achieved, like...
-Store and display all the fluids in your modpack (216 in mine). Attach redstone triggering for < or > half-full levels for automation.
-Build a trophy room with all bosses' heads/items and the weapons you used to kill them.
-Create a gridded automated rail system that expands itself when you want. Figure out a way to quickly map the whole grid out to 5000 blocks in each cardinal direction.
-Fill a museum with all types of certain items in sections: bees, butterflies, wood, hats, mob heads, gemstones, etc...
-Build a hall of travel for all your dimlets, linking books, and other portals.
-Build a zoo with all the mobs in it (except maybe the bosses).
-Build a room where you can quickly swap-out blood magic ritual configurations...using spatial pylons.
-Build a sick arena for all your boss fights. Try different approaches to killing them. Have an alternate mode where a dozen random mobs spawn instead.
-Build a musical apparatus that can play a 100-note section of your favorite tune. Another mode alternates through your discs on a 10 minute timer.
-Take out the UrGhast with a swarm of drones
-Create an adventure map. Example: InceptionCraft - use nested Mystcraft worlds or dimlets, forcing the player to keep going deeper...