They're never going to resolve the Phyrexian, Nicol Bolas or Eldrazi plots, anyways. I can't really fathom them deciding to come back to Theros, either, to be honest.
I think they'll definitely come back to all those things. They've brought them up in supplemental products and story beats too often not to do that. It'll just take forever because of the time frame they work with. I don't know if they'll ever do anything as final as blowing up all the Phyrexians like they did in the old days, but things to do seem to be progressing in some fashion or another.
It'll be interesting to see whether they do it in a "Return to Blah" set or in a set that is set up as "Nicol Bolas is up to no good in Fantasy World Where the Ground is Rubber". Like I doubt we'll go back to New Phyrexia, but the Phyrexians can totally start causing beef elsewhere.