You shouldn't hold your breath for Natalie Portman to return as Jane Foster at any point moving forward in the Marvel Cinematic Universe. If you do so, you'll most likely end up dead, especially in light of Portman's new revelation that she is completely done with all things Marvel as far as she knows.
Speaking with the Wall Street Journal to promote her next film A TALE OF LOVE AND DARKNESS, Portman was asked about any more Marvel involvement to which she replied, “As far as I know, I’m done. I don’t know if maybe one day they’ll ask for an ‘Avengers 7’ or whatever I have no idea. But as far as I know, I’m done.”
We already knew that Portman would not be showing up in THOR: RAGNAROK, but now we can confidently expand that to "ever again." You really never should say never, and who knows if Marvel might come calling again if they have the right story for Foster to show back up… but I'd venture to guess that character is going to be left in the dust as we move forward into future Phases, never to be heard from again.
A TALE OF LOVE AND DARKNESS opens in select theaters on August 19. THOR: RAGNAROK comes our way on November 3, 2017.