Cosmic Nexus Sorcerer

You have a supernatural connection with the numerous planes of existence that make up the physical universe. Perhaps your bloodline was marked by the Primordials, ancient titans of the young universe, or perhaps your ancestors accumulated traces of concentrated energy while travelling through the planes. Perhaps your own essence got entangled with the Elemental Chaos as the result of a magical experiment gone awry. For some reason, your body is constantly resonating with and drawing power from the different elemental forces of the cosmos.

Planar Attunement

Your being is more sensitive to certain cosmic forces depending on the alignment of the planes on that particular day. This results in you gaining Resonance with certain types of energy, Aversion to others and a burst of power called an Elemental Surge:

Resonance: You are resistant to this damage type, and whenever you cast a spell that deals damage of this type you can add an additional Resonance Die to one damage roll of that spell. This Resonance die grows as you gain levels in this class: at 1st level (d4), at 5th level (d6), at 9th level (d8), and at 15th level (d10).

As a bonus action, you can alter the composition of the next spell you cast so that it deals this type of damage instead of its usual one. Secondary effects the spell may have, such as setting objects on fire, are modified at the DM’s discretion.

Aversion: If you take damage of this type, you have disadvantage on attack rolls, ability checks and saving throws until the end of your next turn.

Elemental Surge: This ability has different effects depending on your attuned plane and can be used a number of times equal to your Charisma modifier. You regain all expended uses when you finish a long rest or whenever you change your planar attunement.

Starting at 1st level, you roll a d6 after you finish a long rest to determine your features for the day, with each number on the die corresponding to one of the elemental or echo planes of the cosmos. This planar attunement lasts until you finish another long rest, at which point you roll a d6 again and instantly replace your Resonance, Aversion and Elemental Surges with new ones:

Planar Attunement

Plane	Planar Attunement Features
1. Elemental Plane of Fire	Your skin steams and your eyes glow hot, burning with an inner flame. You gain Resonance with Fire damage and aversion to Cold damage. Fire Surge: Whenever you take damage, you can use your reaction to burst into flames and deal Fire damage equal to your Resonance die to all creatures within 5 feet.
2. Elemental Plane of Water	Your body becomes partially translucent and cool to the touch. You gain Resonance with Cold damage and aversion to Fire damage.  Water Surge: Before you make a Strength, Constitution or Dexterity Saving throw, you can use your reaction to briefly become sturdy ice or shapeless water, adding your Resonance die to the total.
3. Elemental Plane of Air	Your movements become nimble and erratic, energized by an unseen gale. You gain Resonance with Lightning and Thunder damage and aversion to Bludgeoning damage. Air Surge: When you move, you can spend your reaction to blast forward and gain a bonus to your speed until the end of your turn. This bonus is equal to your Resonance die multiplied by 5 in feet.
4. Elemental Plane of Earth	Your flesh becomes hard and unyielding as a boulder. You gain Resonance with Bludgeoning damage and aversion to Thunder damage.  Earth Surge: When you are targeted by an attack or spell, you can use your reaction to instantly encase yourself in stone and add your Resonance die to your AC until the beginning of your next turn.
5. The Feywild	Your spirit radiates mysterious natural energies of the fey echo plane. You gain Resonance with Radiant damage and aversion to Necrotic damage.  Fey Surge: Before you make an Intelligence, Wisdom or Charisma saving throw, you can use your reaction to burst with inner light, adding your Resonance die to the total.
6. The Shadowfell	Your being is sustained by the necromantic forces of undeath and decay that arise from the shadow plane. You gain Resonance with Necrotic damage and aversion to Radiant damage.  Shadow Surge: When you have 2 or more hit points and damage would knock you unconscious, you can use your reaction to channel dark energies from within, dropping to 1 hit point instead and gaining temporary hit points equal to your Resonance die.

Forced Re-Attunement

As an action you can forcibly change your planar attunement to a plane of your choice, instantly replacing your current features with new ones, regaining all your Elemental Surge uses and gaining one level of exhaustion.

Synchronization

Starting at 6th level you can spend a bonus action and 4 sorcery points to enter a state of concentrated energy for 1 minute. For the duration you can transform any spell you cast so that it deals a damage type with which you have Resonance without spending a bonus action. Additionally, every time you roll your resonance die, you can add your Charisma modifier to the result.

Adaptive Transmutation

Starting at level 14, your body transforms to further reflect and endure the nature of your attuned plane:

Fire Form. You become immune to fire damage and can walk on lava as if it were solid ground.

Water Form. You gain a swimming speed equal to your walking speed and can both breathe and speak underwater.

Air Form. You gain a flying speed equal to your walking speed.

Earth Form. You gain a burrowing speed equal to your walking speed through soil or sand and half your walking speed through solid rock or worked stone.

Fey Form. You can use an action to teleport to an unoccupied space that you can see within 30 feet.

Shadow Form. When you are in an area of dim light or darkness, you can use an action to become invisible. This effect lasts until you make an attack, cast a spell, or enter an area of bright light.

Elemental Singularity

Starting at 18th level, you have mastered the balance of elemental energy flowing between the planes and into your body. When you use the Synchronization feature, you become partially attuned to all planes of existence at once, gaining Resonance with all damage types for the duration: you are resistant to all damage, you can make spells you cast deal any type of damage you choose and you can add your Resonance die and your Charisma Modifier to one of their damage rolls.