I'm running a DnD 5e campaign and had a dragon-born sorcerer with Draconic Bloodline as a sorcerous origin that used the level 9 Wish spell to wish to become a "true dragon."
Are there any rules about having dragons as PCs? If so, where? If not, how would you go about making it somewhat balanced?
Background
An excerpt from the wish spell:
You might be able to achieve something beyond the scope of the above examples. State your wish to the DM as precisely as possible. The DM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong. This spell might simply fail, the effect you desire might only be partly achieved, or you might suffer some unforeseen consequence as a result of how you worded the wish.
I was considering going with the "the spell failed" option, but decided to have a little fun with it. I warned him about the chance of it going horribly wrong, but he said it would be out of character for his PC to turn down any opportunity no matter the risk.
So I set up a roll table: 00-10 killed him or similar, 11-35 turned him into an egg or against the party or a few other creative things, 36-50 the spell just failed with all of the negative effects of the wish spell gone wrong, 51-80 gave some cosmetic changes, 81-97 adjusted stats a little (can no longer hold weapons an shields but breath weapon is stronger etc.), 98-99 and it actually worked.
Of course he rolled a 99. I gave him exhaustion and said he needed time to recover from the ordeal, but I really need time to figure out what to do with him.
Best Answer
I would treat this as if the sorcerer had cast True Polymorph (another 9th level spell) and concentrated for the entire duration to make the transformation permanent. Since you subjected the sorcerer to great additional risk to use Wish to become a dragon (and the sorcerer really wished to be a dragon) you should treat this as a change in reality rather than a magical effect that can be dispelled.
True Polymorph allows a creature to be turned into any other creature with the same or less challenge rating. Any of the adult dragons in the monster manual should be appropriate permanent forms for the sorcerer. For flavor he should probably turn into the same type of dragon as the dragon in his draconic heritage for his bloodline.
Since permanently being an adult dragon via True Polymorph isn't expected to break the game, being an adult dragon (which can't be dispelled) shouldn't break the game either.
Wishing at lower levels
A lower level character might discover a ring-of-game-breaking-wishing (or its equivalent) and make this wish. Since True Polymorph isn't a spell of 8th level or lower, characters shouldn't expect that their wish be successful. If the character isn't careful in how they word their wish they could become a stone dragon (a statue), temporarily a dragon, a specific dragon who lives somewhere else or on another plane, a dragon that slowly loses its memories and pre-dragon personality, etc. If they are careful in how they word their wish they can still receive only a partially achieved effect and become a wyrmling (or young dragon if the party is in the 7th-8th level spell levels).