I was actually daydreaming earlier that the encore would be "Blank Space" and "Shake It Off", and that her outfit for the encore would be that outfit with the lace veil + skirt + sleep mask on top, or maybe the black dress she wears while walking down the steps (although that'd be harder to remove really fast). For "Blank Space", a bunch of props from the video would be out on stage (the car, a small mini-mansion set, the blood cake, etc.) while she re-enacted the video with a random guy. They wheel in the door (like from the AMA performance) at the end and she opens it, and they take all the other props off stage.

She does a short little speech about her confidence or whatever (while slowly taking off the pieces of the "Blank Space" outfit), makes a cheesy "shake it off" joke, and starts to sing "Shake It Off" as the dancers featured in the video come through the door. Taylor and them do their thing throughout the performance. Before the sings the bridge, she gives a short thank you to the band, the dancers, her friends, her family, the fans, etc, and then continues with the song as the dancers leave the stage. Right as the final chorus hits, about 10-15 pre-selected fans would come in through the door and dance with her during the final chorus. The extended instrumental plays at the end, Taylor says something like "blah blah, remember to shake it off, blah blah blah, and ALWAYS party like it's 1989!", and then she leaves through the door, while the fans are escorted off stage and the show ends.

Would be amazing, but knowing her, she'll probably just re-do the VMA performance of "Shake It Off" for the encore.