Here is some random bosses that i thinked: Golden Golem It will be a about 20 blocks high,high defense and attack. Golden Golem will be a useful boss to get money,armor and weapons. The Golden Golem will spawn by random,when you got a level of 65 or more,it will grab any metal block and heal itself,when you get closer to him he will swing he hand and smash you dealing about the 1/4 of you health so,stay away from it.When you more than 7 blocks away from him,he will fire yellow beams and causing earthquakes,dealing damage to a 10 blocks in he front and he back.When you finaly killed him,he will drop a large amount of gold and some new brand items. Gold Wand: Fire yellow beams at enemies Golden Ring: Raises you luck at 13 Necro Worm A giant worm made from bones and a fire orb at center of it,the Necro Worm will spawn at random with a chance of 5/40 at day,when you reach at the level 100.Necro Worm will come from the ground,and will hide at it,when it come out,will spawn live skeletons,and shoot some flaming bones at you from it core.Im order to kill it,you need to destroy the bone protection,when it is destroyed,the core will morph in a flying white skull,shooting black smoke at you,and firing a deadly laser when it open the mouth. It will drop Necro related items. Bone Longsword: Very Powerful blade,making a bone song when used. Necromancer Armor: A powerful armor,crafted from bones. Necro Staff: Summon skeleton minions to aid you,use bones from use.(To use it you will need to be equiped with the Necromancer Armor Cthulhupillar The most crazy thing that you will ever seen...The Very Hungry Cthulhupillar! Cthulhupillar is a fusion between a worm and a Cthulhu,that is a fusion between octopus and a dragon. You summon Cthulhupillar placing a flesh bag,crafted with flesh,that you get by killing some monsters. He will breath fire at you and try to eat you with it mouth,some times it will just fly and attack you by surprise.Cthupillar can climb walls,fly,dig and hide in the earth.It`s highly recommended to have end-game gear.Cthulhupillar will drop somethings that he eated. Flesh: A large amount of Flesh Bones: A large amount of Bones Hungry Stomach rain ernegy of monsters,it use blood as power. Blood: A large amount of Blood Cthulhu Soul: Equip it,and you will get wings,and will be able to fly!
The first two bosses seem a bit generic, but I dolike the cthulupillar. Maybe he'd even a have his own special planet biome. Of course, it'd be better if it was extremely rare.