The Shifter

A shift in the space time continuum has caused an entity to appear (I'm too lazy to write more. Just know that it can provide some crazy mobility).

Inner Plane of Existence

You gain an inner plane of existence where you can bring items up to 150 pounds into it every trip to it. This is the plane to which you shift to when you use partial shift. Your base speed is 50 feet in your inner plane.

Shifting Fighter

Whenever you come back from your inner plane, you gain advantage for your next attack roll within your turn. You also gain proficiency with shields and martial weapons.

Partial Shift

You gain the ability to partially shift your form to travel from 1 plane to the other. Your body will be in the real world, but you can move another plane of existence. When you wish to come back, you will teleport to the corresponding location to the real world. If you appear in an obstacle, take 1d10 damage. Also, whenever an enemy attacks you, you can use a reaction to roll a d20 dice. If the number from the dice is greater than or equal to 19, you can choose to either take 1/2 the damage, reflect back 1/4 of the damage, but take full damage, or take full damage, but regain 1d10 hp after you defeat an 1 enemy. Healing increases to 2d10 at 5th level, 3d10 at 15th level, and 4d10 at 20th level. Both these abilities can be used twice every short/long rest. 

Unyielding Force

Whenever you slay a creature, you gain 1 force point. You can use force points to add 1d4 force damage to your attacks when you attack with a melee weapon or when you exit your inner plane. If you use more than 4 force points, you become stunned for 2 turns, and gain another 2 turns stunned every other 4 points added to the attack. Every force point added to your attack also launches your enemy 5ft backwards. If you strike the ground with more than 10 force points used, you can cause a minor earthquake. Additionally, when you roll the d20 for Partial Shift, you can add force points to decrease the number you need to roll to activate the ability. D4 Increases to d8 at level 15, and turns into a d10 at 20th level.

False Copy

If you have anything in your inner plane of existence, you can create a false copy of it as an action. This false copy will acquire all properties of the object. If it is a creature, the copy is under the control of the warlock. This false copy will last 3 turns in combat, and 30 minutes outside of combat.

Complete Shift

You can bring anything in a 15 foot radius into your inner plane instead of having to touch it, and you can completely shift into your plane instead of having your physical body in the real world. The limit to the amount of weight you can carry in your inner plane increases to 1500 pounds, and you can send an enemy into your domain for 1 turn as an action