You're missing the point. There's nothing that says entire quadrants of the galaxy are filled with the Borg or Dominion or whatnot. You're inventing a problem that doesn't exist.
and why should we revisiting gamma and delta quadrant as the focus of the series again? We have already visiting gamma quadrant in DS9 and we have visit delta quadrant in Voyager. It doesn't matter if the whole quadrant has been explored or not, at least in a show; if you want to pursue "boldly gone where no men has gone before" theme. Of course, they can still explore the milky way galaxy off screen. But on screen (on TV again?) where is the novelty in it?
Unless the theme is not about exploration but about galactic war or conflict. Because you will face borg and dominion again if the story revisiting these quadrants.