IF stubborn Vince decides to go that route, I believe it will finally happen at Wrestlemania. WM is expected to be a 2 night affair again and Lashey/Lesnar would be a fantastic headliner for the Saturday night show.
Both seem to be booked for Crown Jewel with Lesnar challenging Reigns and Lashley having a rematch with Goldberg. A match of this magnitude can only take place on a big stage and since it didn't happen at Summerslam like it should have, Wrestlemania is the next best place.
As for Big E, I believe he will just be a transitional champion for a couple of months and hold the title until dropping it to Orton as I strongly believe Orton/Riddle will be the WWE Title match at WM. Big E may get to have a few successful defenses against the likes of Lashley (due to interference from Goldberg), AJ Styles, or a newly drafted Edge.