Raw Magic User

Prerequisite: Character must be a full caster.
Suggestion: This feat shouldn't be achieved through a regular level up, but throught intense in-game studying of raw magic, as a gift from the DM.

Note: This is intended as high risk high reward feature. It was tested. Once. My character died, but we saved the day, so it did it's job XD. Sorry for my bad english in advance. VERY long text ahead.

Studying the Art and the Power all you have found is dissapointment. The Weave is no longer a tool, but a wall in your way to true magic. You desire more and so you study in depth the forces beyond the crystal spheres, turning you gaze upon the Phlogiston. It may be curiosity, pride, arrogance or madness, you now study the volatile Raw Magic.

There are two ways of using this feat: Regular and Maximized.

Regular:

Raw Magic allowes you to cast a spell you dont have spell uses left for, by using a higher spell level slot, even when not having that level's uses left either, but at high costs.

This way of using this feat, requires you to have access to, at least, the next spell level to the one of the spell you are trying to use, for when using Raw Magic, every spell costs one spell level higher. Cantrips become 1st-level spells and 9th-level spells become 10th-level spells. Since the Weave is not being used, it's up to the DM to decide if the 10th-level spells works. This being said, you can use an even higher spell level, as long as the DM is alright by stating that 'your character should have these spell levels', the penalties for using this feat remains the same.
The spells that have upcasting get the appropiate buffs. This also affects cantrips, so if you use Acid Splash with this feat and you are level 3, you get the level 5 damage increase by using a 1st-level spell slot. This is increased by each spell slot higher than 0 that is used. If you are level 5 and use this feat, Acid Splash does 3d6 of damage as if you were level 11, but if you are level 5 and decide to use a 2nd-level spell slot it does 4d6 damage, as if you were level 17. This can be 'abused', if you are level 17 you can still use this upcasting, so if you use a 4th-level spell slot, Acid Splash does 8d6 damage.
This being said, if for example, you use a 1st-level spell which has upcasting at level 3, but not level 2, you may cast with this feature using a 2nd spell level, but it wouldn't get any buffs.

If the spell requires concentration and has upcasting the penalties of using this feat activate AFTER the concentration is broken or the spell ends. In this period of time, the character may move and perform actions as normally, but cannot cast any other spell or the concentration will be broken. If the spell does not have upcasting, then you don't need to maintain concentration, the spell just casts for the duration without the need to make checks when you are hit, but the penalties activate immediately.

Now to the fun part.

Roll a D20 with a DC stated by the DM (I leave it at their judgement depending on the spell you want to cast). You may add you Spellcasting Ability Modifier to the roll, and any buffs or debuffs that you character has. If the roll fails you loose the spell use (if any left, and remember, at least one spell level higher than the casted spell's), and you are left with half your actual hit points rounded down. These are the only penalties you are affected by in this scenario.
If the roll succeds you need to roll a natural D20, which cannot be affected by any buffs or debuffs of any kind, Lucky feat included. Depending on the roll's number the results will be the next:

18-20: The Roll succeds and you cast the spell normally, adding the appropiate upcasting.
11-17: The Roll fails and a mild or moderate catastrophe happens.                                          
1-10: The Roll fails and a great catastrophe happens.                                                                 

I suggest the DM to have a wild magic table or another 'terrible situation' table of sorts ready for this moment. It is up to DM to decide if a 1 means a magical nuke. Whatever the result of this roll may be, as you are using such inmense source of chaotic power, you recieve these penalties:

1. You are left with 1 hit point unless you were below 20% rounded up of your total hit points, then you are left with 0 an need to make the death saving throws.
2. You loose the ability to use the spell level you casted the spell with, even with this feat, until the third long rest you make. Remember, if you used a 1st-level spell, you at least used a 2nd-level spell use, so you loose the 2nd-level spell uses, not the 1st-level's.
3. You gain 3 exhaustion levels if you are using the original 5E rules. If you are using the new exhaustion rules from One DnD UA, then you get 6 levels of exahustion.

If the two rolls succeed you may cast the spell normally as stated in the table. This means that even if the two rolls suceed, you need to make the base rolls, or the enemy the spell's saving throws, so be wary desperate caster, you may still fail the spell. This works for both Regular and Maximized versions of the spell.

Maximized:

Raw Magic allowes you to cast a spell's most powerfull form without the access to that spell level. The player need to make the same two rolls from the Regular version. This time, if you fail the first roll, you recieve the same penalties as if you had suceeded in the previous way of using this feat. The second roll requires you to achieve a natural 20, anything else will provoque a great catastrophe. Either way, the penalties for the first roll's success are the next:

1. You gain 6 exahustion levels per the rules of the original 5E, or 11 levels per the rules of One DnD UA.
2. If the character is brought back to life in anyway, you need to roll 1d4. The result is the number of days you character cannot use any kind of magic, including this feat.

If the character is at least level 18, you can use the Maximized version to cast a spell from previous editions of DnD, as long as you character both knows the spell and has all the requirements for it.

As an Epic Boon, at level 20 you can use the Maximized version to cast a 10th level or higher spell. In this scenario, if you character is brought back to life, you roll a 1d20 instead of a d4, and you cannot use this Epic Boon until three in-game months go by. If you try to use this Epic Boon at a level lesser than 20, your soul is completely destroyed and can only be brought back to life with a Wish. If brought back to life you need to roll 1d100. If you get a 1 your character has lost the ability to use magic completely.