Okay so there is few dungeons that are decently hard but you should make a dungeon that is extremely long and difficult and you should add new creatures like piranhas, octopus’s, new boss kraken, sharks, electric eels,etc and a new concept electricity
Piranhas: these new creatures can be found in the rough oceans and the new ocean dungeon that’s is completely submerged underwater they will swim around and will flee if they see you and your alone but if you attack one more will appear and swarm you and eventually eat you and if they already swarming and spot you they will also attack instead of fleeing
Octopus: these creatures are extremely dangerous if they see a player they will fog up the area with ink and attack player by grabbing on to them and attacking them players must knock them off to stop taking damage they are said to be so smart that they have been seen to grab players and throw them into spikes…
Electricity: a new concept if you get hit my anything electrical you will get zapped stunning you for a few seconds new electric bomb spawns a storm cloud that shoots down lightning where the bomb landed it may be dark in some parts of the dungeon so the new electric sound googles may be useful they send out waves of electricity to show your surrounding which show outlines and will show glowing dots of where it detects something moving like a player or mob
Electric eels: this creatures are menaces they will shoot electricity at players causing them to become paralyzed for a few seconds and if you touch them you become paralyzed for a longer duration