Is there anything that is not accessible to the human experience and/or imagination?

Yes, the infinite things: Infinite Love, Infinite Power, Infinite Beauty, Infinite Presence, an infinitely-dimensional universe, etc. We are finite creatures who tend to find the infinite incomprehensible.

Absolutely no.

Humans can experience everything in the world which is accessible and can imagine those which are not accessible.

Potential of human mind is unlimited.

You, as a human, can imagine magical powers will clean you house. Alas, flying without the aide of mechanics and imagining powers of levitation or winking your nose at filth will not happen. There is a limit to imagination because even though we can conceptualize infinity, we don't have the ability to KNOW infinity. We are limited to human thought and the reality that surrounds us.

Any example given is able to be given precisely because it is accessible to human experience in some way. There is no reason to suggest that what human experience detects is all there is. There might be reason to suggest that we ought to think as though what human experience detects is all there is.

If we imagine all of our imaginations together could we imagine ourselves to be anything more than just a little shining object way off in the distance.

Whatever is not accessible.

Related: "Light Is a Living Spirit," "The Path of the Higher Self."

"Non-human-ness." That which is outside the human condition, is beyond are capacity to experience. A human cannot know what it means to be a cat, only know "about" it, through identifying what it means to be a human. As humans, we only experience our senses, as they filter and interpret the external world through perception. We don't ever experience a thing itself, only a sensory translation, which is limited and fallible.

Yes. Infinity.

Of course.

The square root of -1, for example.

We are unlimited.

The only thing not possible is to re-grow a limb you were born without. (why that is the one impossible thing, I do not know).

Everything else is possible.