Is there any synonym for "abyss" with a non-negative connotation?

I think infinite depth is pretty neutral.

Depending on your context, there are some geographic terms that have similar meanings to abyss (maybe without the "depth" aspect, though): ravine, gorge, canyon.

A comment from OP suggests he's looking for a neutral/positive word to describe metaphoric "depth" as applied to a person, in which context I suggest the person is perhaps...

profound - having intellectual depth and insight; characterized by intensity of feeling or quality.

The opposite is often expressed as shallow - lacking depth of intellect, emotion, or knowledge.

It's a little archaic/formal, but you can also say that someone "lacks bottom" (meaning they lack roots, grounding, or principles, rather than lacking gravitas/profundity). I've rarely if ever seen the non-negated form (it's invariably he lacks bottom, not he has bottom).

Could be 'vortex', if you mean some emptiness that causes a 'vertigo' sensation (being attracted to the void, or feeling like you could be falling). You could use 'precipice' as well. It would be nice to have some more clues about what is the context, so that it would be possible to make a more precise comment/suggestion on this.

In Milton I believe the Abyss is anterior to Hell,

Into this wild Abyss/ The womb of Nature, and perhaps her grave--

Perhaps the most positive 20th century association with an abyss is Georg Lukacs' Grand Hotel Abyss - pleasantly perched alongside "the womb of Nature," a little like the Columbia Gorge Hotel in Oregon, but with a picture of Schopenhauer instead of Rudolf Valentino in the lobby:

A considerable part of the leading German intelligentsia, including Adorno, have taken up residence in the ‘Grand Hotel Abyss’ which I described in connection with my critique of Schopenhauer as ‘a beautiful hotel, equipped with every comfort, on the edge of an abyss, of nothingness, of absurdity. And the daily contemplation of the abyss between excellent meals or artistic entertainments, can only heighten the enjoyment of the subtle comforts offered.’

In my opinion the word zenith may well explain the opposite of abyss. As abyss means very deep shasm, metaphorically related to very depressing emotions, the word Zenith is the highest point on the celestial sphere infinitely high as the climax but again metaphorically represent very high flying and positive state of mind. in Urdu Zeenet means ultimate beauty.

How about fathomless and unfathomable? Eg: The beauty of God's creation is indeed unfathomable; meaning it's too profound to fully understand it for mankind.

Found from a book by Daniel J. Siegel, there is a phrase called "the sea of potential". This covers the positive key words of "Profundity" and "Expanse" that one would find in a sentient mind that also connects with the depths of the ocean, and the expanse of the Multiverse (multiple universes). "Profound" suggests something discovered and brought to common knowledge, while the expanse of the universes are undiscovered and found to be the mysterious and great beyond of common thought processes.

The "Sea of Potential" is more of a verb phrase, given that potential energy turns to kinetic energy when acted upon by another force. We live in a constantly flowing, kinetic, wave-like ocean of happenings. To bring some peace into the ever constant change that occurs, we humans create patterns that keep us centered on our home base planet. In other words, we build relationships to guide us on the "sea of potential".