Long ago, when Notch was still apart of Mojang, he planned to add a Red Dragon. Sadly, it was forgotten, and as a result, never added. That could all change eventually, anything could happen. Just in case the Red Dragon is added, I have some ideas.
The Red Dragons base color should be #9e2700. The model of the dragon should be a little different and more complex than the Ender Dragon. The size of the Red Dragon should be 3/4 the size of the Ender Dragon.
The Red Dragon would be extremely rare, only 3 should spawn in a world. They spawn in their own underground structure that generates between Y=45 and Y=20. When you find the Red Dragon it's asleep. In order to wake it up, you either have to hold the Dragon Egg in your hand, which would make it passive towards you or if you steal it's loot, doing that would make it hostile.
When angered the Red Dragon has 125 HP and drops a bunch of loot like lapis, diamonds, emerald, and golden apples (Enchanted and Unenchanted). Without full diamond armor the dragon does 7 hearts of damage, and with full unenchanted diamond armor, it does 3. It is immune to fire, and explosions.
Being Tamed/Tamed Abilities
The Red Dragon is a tameable mob and is tamed with the rarest item in the game, the dragon egg. When tamed, you have the option to ride it. Once on the dragon, you could fly it out of the cave, and doing that would unlock its abilities. Its abilities are shooting fireballs, going through blocks except for obsidian, anything made with obsidian, bedrock, and blocks of ore. It'll have 175 HP when tamed, and will attack anything you attack and would deal 9 hearts of damage.
The Red Dragon can wear armor, but only dragon armor. Dragon armor is found very rarely in end cities, but that's the only place you can find dragon armor. There are diamond, gold, iron, and leather dragon armors. Also, in the end cities, you have a 0.1% chance to find the dragon tracker, which is a map to the Red Dragons structure.
Benefits: Nobody would be upset with this addition, it adds a use for the dragon egg beside decoration, and it's the biggest reward you could get from defeating the Ender Dragon, it might seem OP, but it's extremely rare.
Remember, if you want this to be a feature in vanilla Minecraft, give it your vote, or if you have ideas for this post, comment them down below.