Cobaya Sorcerer

You have been the subject of experimental magic, either in your early youth (before you can remember), or as a child or teenager. The experiments infused your body with abnormal magical capabilities, which are often beyond your understanding or control. The goal of the experiments could be a plot to create an army of artificial Sorcerers, or something else you come up with. However, in your own experience, something went wrong and you were either discarded or you managed to run away. Only the spellcasters directly involved in the experiment might know something about what you can do, however even them disregard your potential and only slightly fear the repercussions. You are not their first, nor last of their failures.

Your spellcasting ability is Intelligence instead of Charisma, as most Cobayas end up not trusting their own magic and try to rationalize everything around them in a search for answers. Your high inteligence could come from studying, like a Wizard, or a side-effect of the experiments, giving you sudden bursts of insight as you navigate the Weave with your Magic. 

Due to the nature of being dismissed as a potentialy unreliable spellcaster, and in general having low self-esteem towards their powers, Cobayas often have to adapt and survive by other means, giving them increased resilience in battle that otherwise wouldn't exist for normal magic users.

Most Cobayas have a mark, brand or other sign as a result of having been a test subject. It could be a number or a symbol of the organization behind the experiments, tattooed or etched somewhere in your body. It could also be an awful scar, deformity or magically innert embedded fragment , a constant reminder to you, and others, of your painful history. Work with your DM how to best incorporate this into your background narrative.

Adaptive resilience

You gained incredible resilience by not trusting your magical capabilities and restraining yourself from relying on it for survival. Or, you could have focused most of your magic inwards, in an attempt to avoid unwanted destruction with an offensive action, allowing it to naturally coalesce in a permanent defensive mechanism for survival. Whatever reason you choose, this feature sets your base armor at 13.

At higher levels: At 11th and 16th levels, you gain an additional +2 to your base armor. 

Bold Intent

Your powers give you increased confidence in yourself and your capabilities as a magic user.

At 7th level, you may choose to give up +2 AC from Adaptive resilience for an extra spell slot of 2nd level, or 2 spell slots of 1st level, as a result of being more familiar and more confortable with your spell casting. 

Reckless Enthusiasm

Your experience in battle propels you to take even greater risks with your magic. You draw more magical power from your innate defensive mechanism.

At 16th level, you may choose to exchange another +2 AC for an extra spell slot of either 6th, 5th or 4th levels, 2 spell slots of 3rd or 2nd levels, or 4 spell slots of 1st level.

Rationalist

The nature of your powers urged you to seek answers in a more rational way, looking for patterns, investigating your actions and thoughts, even generating theories from the results of your spells. You have a +1 bonus to your Intelligence modifier. The nature of intense thinking tends to turn the thinker against himself in a harsh, often unjust self judging behavior. Your Charisma modifier is reduced by 1.

Dark Mirror

You catch glimpses of a person through the Weave. You think it might be you, had you not been subjected to the experiments that turned you into what you are, or had them not gone terribly wrong. You often commune with this "other" self, and learn from it.  

At 6th level, you gain the following abilities:

Unfettered Existence: Every time you make an ability check, saving throw, or attack roll you may spend a sorcery point to draw knowledge from your "other existences" and roll an additional dice, depending on your proficiency. You must call it before making the first roll. If you have the Sage background, you may also add advantage to your History checks.

Spy of the Weave: On attack rolls, you may spend 3 sorcery points to roll another d20 for a much higher, albeit risky, chance of a critical hit.

Tactics of Mistake: If you cast a spell that requires an enemy to make a saving throw, you may spend two sorcery points to change the type of save required.

"When the Weave teaches, listen.

If the Weave scolds, accept.

Should the Weave reward, reap." 

El'ith Vilthranor.

Unshackled

The world at large recognizes your abilities as either something to be feared, or admired.

At 14th level, you may decide if you are to be treated as a magical wonder, with Eldath's Path of Redemption, or a dangerous beast, through Azuth's Path of Hazardry.

At 19th level, you may further solidify your reputation as either a mystical hero, by becoming a Scion of the Weave,  or an unstable entity of tremendous power, by embracing your fate as a Freak of Nature.

These features should reflect your choices during the campaign.

Azuth's Path of Hazardry

The image of "dangerous liability" never left your persona. As you grew in reputation, so did your power, and the tales of what it could do. You decided it was more important to explore your own limitations than to appease a fearful, superstitious crowd. Where can this magic take you? There's only way to find out.

Rythmic Crescendo: Choose between adding a +2 to your Intelligence or Constitution modifiers. Whenever you make an Intelligence based check, or save, you may add a d4 to that roll. This grows consecutively through all dice up to a d20, as long as you are sucessful. The dice grows only if the check was of moderate or higher difficulty. Failure on a d12 or d20 will reset the dice to a d4 and require a Constitution saving throw to avoid exhaustion. You are considered to be concentrating for as long as you hold a die above a d6.

Reverberation: Every time you spend one or more sorcery points, you may use your reaction to roll a spell attack. It deals 1d8+5 damage per sorcery point (maximum of 5) to up to 5 random hostile targets within 60 feet of you. Half that damage then spreads to surrounding creatures of up to 10 feet.

Eldath's Path of Redemption

Your knowledge of the Weave has reformed your reputation and graced you with a redemptive road upon which to tread. You aren't necessarily a legend, yet, nor a force for good, unless you wish to walk towards it in the future. You feel that you can finally make that decision.

Cosmic Alignment: You get a +2 to your Charisma modifier. You become proficient in the Persuasion, Deception and Intimidation skills, if you aren't already. You also have advantage on Charisma saving throws.

Mystral Embrace: You may roll a d20 at the start of every turn. You may pick any roll, and keep it. You can only keep one at a time. Once per long rest, you may use that roll to replace any other d20 roll you make. You can only use this in battle.

Freak of Nature

Exile is what they wish for someone, or "something" like you. It's not so much that they fear what they do not understand. They cannot properly fear what they do not understand. And they should. Everybody should. Perhaps, including yourself. But your tireless hunt for knowledge has brought you closer than you could ever have expected to true understanding. You don't care about the costs.

The limits of your existence are no longer bound by the Material plane. Well, any plane. You have become something with the Weave, and whatever it is, the repercussions are just now beginning to be revealed.

Epiphany: Your Intelligence score is increased by +2 and it is no longer capped at 20. For every point above 20, you gain a penalty of -1 on another Ability Score of your choosing. An ability score cannot go lower than 3 by this mechanism.

Whenever you make a saving throw or a check that is not Intelligence based, you may add some or all of your Intelligence modifier by spending sorcery points equal to that amount. Whenever someone else that is proficient in a skill is required to make a check, you may spend a sorcery point to also do it when you otherwise couldn't. You are temporarily considered proficient in it, but you don't add your ability modifier in that check.

Unfiltered Sorcery: Whenever you cast a spell, you may spend sorcery points to alter its die and damage type.

Reign of Chaos: As an action, you may activate an Aura. Your Charisma modifier is set to 0 and your movement speed is reduced by half. For the duration, you are immune to any conditions and gain temporary hitpoints equal to 5d10 x your Intelligence modifier. Every turn, creatures in a 60 feet radius around you must make a Charisma saving throw or take 2d10 psychic damage.

Your Strength ability score is set to 18.

You may cast any spell as a bonus action.

Scion of the Weave

"You are an heir of Mystra herself." You may not believe these words literally, but they echo within your mind with an ever growing presence. Your path has lead you to a position within yourself seldom spoken of, let alone presenced. It is not your reputation that impresses you, but how well you have gotten to know yourself, and to trust it. You have become one with the Weave, and in doing so at the same time lost and gained yourself. You are both you, and not you.

Telluric Self: You may spend a number of sorcery points equal to whatever number you require to pass any Charisma check or saving throw. You may use this ability after rolling the required dice. You are also immune to being Charmed.

Telluric Presence: Whenever you try the Persuasion, Intimidation or Deception on another character or creature, if your total is above 20, you may choose to spend 3 sorcery points to Charm that character or creature instead. The character or creature does not make a saving throw, and remains Charmed for up to 8 hours, or until you decide to end it with an action or it takes damage from you or your allies.

Aura Cangiante: As an action, you get +2 to your AC, and you gain the ability of Detect Magic and Identify for the duration. You can react to creatures targeting spells at you before they can finish casting them.

Gift Bearer

The Weave has become part of your existence to an unprecedented degree. You feel its magic all around and within you, and with your superior understanding of it, you can channel its powers to a degree rarely seen from other Sorcerers. Those who glimpse your power call you "Gift Bearer", and you start to feel that it might be an apt description of your abilities.

Inscrutable Sense: You may spend 6 Sorcery points to roll double your spell dice (damage or otherwise), or 12 Sorcery points to roll triple your spell dice.

You may not alter your magic with an additional effect if you use this feature.

Arcanist Focus: You may recover all your Sorcery points by focusing on the Weave for one turn. You cannot move, speak, take an action, a bonus action or a free action, and you can't take reactions until the beginning of your next turn. These points are temporary, and only last until the end of your next turn.

Sight of the Demiurge: Whenever you use Arcanist Focus or Inscrutable Sense, you gain access to every spell from all Arcane magic schools. This effect lasts until you cast any spell.

You may not alter your magic with an additional effect if you use this feature.

Counterweave: As a reaction, you may use an improved version of Counterspell at any hostile spells being cast directly at you. You recover Sorcery points whenever you do.