Incorporeal: You have no true physical body. You can not have/use ANY weapons or clothing, and you can not physically interact with sentient beings unless they are incapacitated, charmed, feared, grappled or prone, or unless they consent. Carrying Capacity is unaffected, and is treated as though you have a 10 in strength. However, you have resistance to all non-magical damage, and resistance to all necrotic damage, magic or not, and can not be grappled or hindered by non-magical movement-altering effects.

Inconsistent: While your size is normally Gargantuan, you can change to any size down to medium. Every stat except AC/Special Attack modifier (Treated as if medium) and Carrying Capacity Modifier (Treated as if Gargantuan) is correctly adjusted.

Incomprehensible Mind: You are immune to being Charmed or Frightened, and resistant to Psychic damage. Additionally, you gain the effects of a long rest from a short rest, but you can only gain a short rest worth of health back, even in a long rest.

Uncanny: Your creature type is Aberration, and you are considered Otherworldly (able to be targeted by Banishment).

Malleable: Instead of having permanent proficiencies, you have 8 exchangeable proficiencies and 3 exchangeable expertise. You may change what these are after a short or long rest. You can gain NO MORE proficiencies, but can gain more expertise.

Far Realm Resilience: Your unarmored AC is 10+ Dex. modifier + Cha. Modifier. If Charisma is your chosen max stat, the armor bonus you gain from Charisma is locked to +5.

Potentially more W.I.P.

You can establish a link to a sentient being within 120 feet of you in three different stages. A target(s) can be linked to if they are charmed, feared, incapacitated (any form), grappled or prone (Must roll one of your non-30 mental stats, at advantage), if they consent (no roll needed), or if they are actively rolling death saves (no roll needed). You must stop using one form to use another.

Mind Link: Free Action - You can telepathically link to up to 3 different beings at one time. You do not have to share a language, but they must know at least one language. If telpathically linked, your resistance to psychic becomes an immunity. A Mind Link lasts 1 hour, or if you disconnect the link yourself, using an action.

Spirit Guardian: Action - Your body disappears, as you become attached to a being spiritually. You can not be targeted, and you take a turn at the same time as the person you are attached to. You can not move, as your link moves you along with them, but you can act, and any support spells/traits that you use apply to your link as well as you. Additionally, you can cast guidance for free, but only on your link. You can exit your link as an action, and you can get forced out if your link takes ANY damage from a magical source. 

Full Possession: Action - You fully possess a being's body. The being is unable to act, as you have complete control. Every one of your hosts physical attributes (STR, CON, DEX, physical  features, AC, Hit points, size, speed, etc.) are transferred to you, but all of your mental attributes remain. Carrying capacity, AC and special attack bonuses are not changed. You are no longer resistant to non-magical damage. However, your resistance to Necrotic damage is upgraded to full immunity. While your carrying capacity is not affected by your host's size, it now takes an action to pull any item from out of your true body. Only items that the host has in its inventory do not take actions to pull out. You use the weapons/armor of your host, and lose the effects of Far Realm Resilience. Your spellcasting remains the same. All damage taken is given to the host. If the host reaches 0 HP, you are forcefully ejected from the body. If this happens, you gain disadvantage on ALL ROLLS until you complete a rest. You can leave your host with an action at any time.