Homebrew Ancient Dragon (playable) Species Details

You are more powerful than any other race native to the material plane that has not been altered by magic. You live forever, unless killed.  You have the ability to transform into anything you imagine with some restrictions, you don't need to have seen your transformation before; ie: if you think about being a loxodon that is very strong, you can be a loxodon, but it's strength modifier can't be more than double yours. You can make all your ability scores double, and that would be fine, and that would be like your mega evolution. But you can't turn into a god, gain immunity to something, transform into an extraplanar creature that has a challenge rating higher than 20, turn into an inanimate object, turn into something indestructible or hard to kill, and you can't turn into something that defies logic. (that means that you can't turn into the force of nature or cthulhu, or a thing that is unfathomable by a human.) it takes a bonus action to transform and when you die in your new form you come back to your dragon form and be prone. If there is any boundaries that restrict you from transforming you appear in the nearest possible area that you can fit in and you can't transform into another creature for 1d4 hours.

Ancient Dragon (playable) Traits

You have a +5 to all stats, and your maximum is now 40 for those scores, your breath weapon is the same as indicated in the monster manual, and your AC is 20 plus your dex modifier.

draconic ancestry

Because of you being an ancient dragon, you gain a +5 to all stats, the maximum of these numbers is 40 not 20. You gain the breath weapon that is stated for your color or metal type in the monster manual. You also gain AC because of your scales and that  is 20 plus your dexterity modifier.