UA Psionic Wizards get a special ability called Thought Form where they become beings of pure psionic energy.
While you are carrying your psionic focus, you
can use a bonus action to magically transform
your body into pure psionic energy. The
transformation lasts for 10 minutes, until you
use a bonus action to assume your normal form,
or until you are incapacitated or die.
While in thought form, you are a figure of
luminous psychic energy, with your psionic
focus hovering within. Your form can appear as
anything you wish, but it is obviously magical, is
the same size as you, and sheds dim light in a 5-
foot-radius. Any other equipment you are
wearing or carrying transforms with you and
melds into your thought form.
Can you still physically interact with the world in Thought Form? Eg, can you open doors, pick up items, attack with weapons?
Would you still need to breath? Seems unlikely that you could breath, especially since you can take nearly any form.
What happens if you are hit with Dispel Magic?
Is your form fluid, or fixed by whatever you first appeared as?
Thought Form only has the specific effects it lists, D&D 5e does not have hidden rules
You are still a creature and interact with the world as one. Being pure psionic energy
is not exclusive to being a creature and interacting with objects in the world.
The feature does not say you don't have to breathe, so you do.
It is a class feature not a spell. Dispel Magic only ends spells:
Any spell of 3rd level or lower on the target ends.
Whether you can change your form is a bit ambiguous as the feature does not state you can change it (nor how or when), but is not specific about how you choose it and explicitly lets it be whatever you want. It is perfectly reasonable for your DM to say it has to be fixed (following the idea that it is the psionic representation of you), or let you change it as you wish. Ask your DM, and possibly send feedback to WotC that you are unclear of how this part of it is supposed to work (so it can possibly be addressed prior to any final publication).