EzekielRaiden
Follower of the Way
Exactly as @Oligopsony said.
Why do you want to play this? What do you need from it to feel satisfied by it? Are you prepared to start off with few resources or powers and grow into them over time?
For example, if the only thing you care about is "I can cast both healing spells and fireball," consider playing a Wildfire Druid or Light Cleric. These are both characters that can heal and cast fire spells (though not specifically fireball, at least for the Wildfire Druid) Personally, I'm quite partial to the Wildfire Druid, and think the Light Cleric is kind of a dud; I would be willing to work something out so you could learn fireball as a Wildfire Druid if that specific spell is super important to you.
On the other hand, if your interest is simply "I want to be that powerful thing because it is powerful, no other reason," well...we're going to have a problem. That argument does not sell me on the concept. I need to be sold on it. I need to see that sincere enthusiasm, because that's a huge driver of my GMing, how I'm able to deliver exciting situations and deep conundrums.
On the gripping hand, perhaps you have an idea for a character who has discovered a unique magical tradition that draws out both the cleansing and destroying power of flame. We could work together to develop a Sorcerer subclass that blends some healing spells with fire-based offense spells. Or we could make a custom Warlock patron (probably using the Celestial patron as a starting point) that offers features which support such a concept, the "Soothing Flame" or something like that. A Tome Pact Soothing Flame Warlock could be a very good fit, if you have interest in pursuing the story of someone torn between healing and destruction.
I would let players play a depowered red dragon if they were sincere about doing so because it's what excites them, not because they're trying to do an end-run on the system. I'd let them play an honest-to-God balor, cursed with weakness until it proves itself a hero (that's a juicy story, and I can take leaves from Cthulhu Saves the World for it). I'd let a player play a "reformed" mind flayer that has sacrificed everything, including its greatest psionic powers, in order to be the first of their kind to live without needing to murder innocent sapient beings, slowly working their way up to becoming a new Elder Brain that can challenge mind flayer society from within. Etc.
But only so long as the player is sincerely enthusiastic--meaning, they aren't being exploitative, abusive, or coercive with their requests or behavior.
