In order to get Sigourney as Ripley back in Covenant 2 would need some very youthful stage makeup, and it would need some explaining for her ageing, plus she would need to be up for lots of running through corridors and prancing about.
Getting another actor to play Ripley in another movie could be possible, but very risky. To me, Sigourney = Ripley, and Ripley = Sigourney. She would fit much better in a post-Aliens Blomkamp film, rather than a Covenant 2 or 3.
Given that Ripley was chest-bursted and last seen falling into a furnace in Alien 3, I cannot see her surviving by some weird split second plot twist as she fell. So her insertion back into the franchise can only be between Aliens and Alien 3 timeline.