Does this seem balanced as an alternative Capstone ability?
Elemental Ascension (Ex)
At 20th level, you transcend mortal limits, becoming a living embodiment of your primary element.
You can accept 1 point of burn to transform into a Medium Elemental of your primary element, as if using Wild Shape. While in this form you retain your ability scores, skills, and class features, and you can use all kineticist abilities, including wild talents, while transformed. You also gain Darkvision 60 ft and Immunity to bleed, paralysis, poison, sleep, and stunning.
If you have the Kinetic Form utility wild talent, you can activate it while in elemental form, letting you transform into a Huge Elemental of your primary element.
Once you have accepted burn to activate this ability, you can freely shift between your elemental form and your normal form as a standard action until your burn is removed. This ability is a polymorph effect, and any other polymorph effect applied to you will override your elemental form until it ends, at which point you can resume your transformation if this ability is still active.
IMO. it seems a little weak. A druid can shift into a Huge Elemental at 12th level, so I would expect a Capstone ability to grant more than just a Medium elemental.
I think I'd just remove its native burn cost and call it a day. It's a convenience ability at a level where most of the convenience can be covered by other means.