we have very little to go on, starwars.wikia.com/wiki/Unknown_regions maybe?

My guess would be the Unknown Regions. The mystical nature of the Rebels ending gave off a vibe that they'd be going to parts unknown.

The Aftermath trilogy and Thrawn novels have given a lot of hints about mysteries that lay beyond the known galaxy. That seems like fertile storytelling ground for Thrawn and Ezra, and eventually Ahsoka and Sabine.

especially considering Thrawn is always associated with the UR