Note: This is not canon, so don't except this to be added any time soon, or at all.
Credit to JustAnotherBoi.exe for the idea of an item that creates stronger dragons.
This is my interpretation of the ice and fire bosses, as well as a lightning boss I came up with a while ago.
Ice Boss, the Dread Queen and Dread Guardian[]
After entering the dread portal, there is a huge blizzard surrounding the spawn area. Waves of dread minions come from the blizzard, with the Dread Queen and Black Frost at the end. The Dread Queen drops her sword or staff. The staff can summon up to three random dread minions at once. If the player dies during the battle, it will be restarted once they enter the portal again. Every time the player enters a previously unentered portal the dread queen battle will start again. Maybe it can be manually started with a special item.
Once the dread queen is defeated, the dreadlands can be completely accessed. There are dungeons and caves with previously unseen dread creatures, like dread brutes, bulkier ghouls, and dread goliaths, massive beasts that could be considered minibosses. Dreadsteel ore can be found in these caves, which is similar to netherite and can be used to upgrade diamond armor. Dreadsteel armor gives the player immunity to catching on fire. Standing in fire or lava will still damage the player, but they can't catch on fire while wearing the full armor set. Dreadsteel can be turned into dragon armor.
Dread castles appear on the surface of the dreadlands. These castles are composed of rooms with dread spawners in various places, as well as chests with powerful loot. At the top of the castle, there is a locked chest on the dread throne, but as soon as the player approaches it, two stone dragon heads shoot dread fire (black fire) and summon the dread guardian, a powerful dreadstone automaton with a glowing eye and a blue gemstone embedded in its chest. It wields a hammer that can create spikes of ice to damage the player. It will drop the gemstone, used to make ice dragons more powerful when equipped, and it drops the key to the locked chest as well as its hammer. The chest contains the undead summoner, which can be used in the overworld to create a tamed undead stage one dragon that can grow and breathe dread fire. The summoner is consumed after it has been used once.
Fire Boss, the Ifrit[]
The Ifrit will spawn in a large, primordial ring, which is primarily made of nether bricks, but the inside is made of netherrack with a small pool of lava in the center. The Ifrit floats on the lava, slashing at players with his sword and shooting bursts of fire at them. He also destroys some of the netherrack, leaving holes of lava for the player to fall through if they aren't careful. After he runs out of HP, he sinks beneath the lava, breaking the remaining netherrack, and bursts out in the form of a lava serpent with a large red gemstone embedded on its forehead. The serpent can bite players with its lava fangs, breath fire, and also summon blazes over the lava pit. Once defeated it will drop the gemstone, which is used to make fire dragons more powerful, and it will also drop the Ifrit's lava stone, which grants the player pyrokinesis. At this point all of the blazes die, and the lava in the arena hardens into obsidian.
Lightning Boss, the Midnight[]
Legend tells of a giant bedrock chain leading to the edge of the sky itself. If one were to climb this chain, they would find barely anything at the top, except for some strange runic blocks. But during a thunderstorm at midnight, a massive tear in the sky manifests, and once entered, the player is brought to the prison of the Midnight.
The Midnight is an enormous lightning dragon, about the size of a stage 6 or 7 dragon. It has pitch black scales, streaks of glowing purple slashed across its body, and a massive purple gemstone embedded in its chest. It is chained to a tower atop a floating island, existing in the depths of the void. The dragon will fly around the island, summoning bolts of lighting to strike the player, which must be dodged quickly. It is immune to all forms of damage in this phase, and the player must wait for it to shoot a ball of electricity at them. This ball can be hit like a ghast fireball, and when it hits the Midnight, the dragon will crash onto the island, vulnerable to attacks from the player. This will repeat until the Midnight runs out of HP.
Once the first phase is over, the Midnight will fly to the center of the island. The gemstone on its chest will begin to glow brightly as the dragon absorbs incredible lightning power, destroying the tower and transforming the arena. Several dark pillars appear on the island (similar to the ender dragon battle), and they create a shield around the Midnight. The Midnight will begin healing the damage it took in the first phase, summoning bolts of lighting and electric blazes to attack the player. The player must scale the pillars, hitting the crystals at the top. Once hit, the crystals will change color and stop feeding the shield. Once all of them have been hit, they will fire at the shield, destroying it and stopping the Midnight from healing. The Midnight then falls onto the island and enters the third and final phase.
In the third phase, the Midnight only has the health it absorbed from the crystals. It fights like a regular dragon, except it is slower with stronger attacks. It can also summon more bolts of lightning from the sky and shoot massive electric beams at the player. It will also shoot electricity balls that can be knocked back, stunning the dragon for a time. Once it runs out of HP, light erupts from its body and it dissolves in a brilliant explosion. It drops stacks of lightning dragon scales, along with some blood, a special dragon heart, and its crystal. The crystal can be used to make lightning dragons stronger, and the heart can be crafted into a staff that can summon the Midnight's bolts of lightning. Thundersteel ore appears on the island, which can be used to upgrade diamond. Thundersteel armor gives the player significant resistance to lightning (including dragon lightning), and the sword has greatly increased knockback. A portal will appear, taking the player back to the overworld and resetting the entire island. The chain can be entered again, allowing the player to fight the Midnight as many times as they desire.