The 707 had it's first flight 60 years ago in 1957. At their present production schedules, Boeing will have tapped out the entire 7X7 series within the next 40 years by 2057. Obviously, they are going to continue making airplanes after that date, but what exactly. Based upon existing trends I believe it's safe to say whatever comes next would have:

- hybrid wing bodies (see pic/vid related) which Boeing has experience with through the X-45, X-46 and their recent MQ-25 entry
- hybrid propulsion (as in tail gas engines powering electric propellers via an alternator) which NASA is currently researching via the X-57
- unmanned piloting available as an option for freighters
- opening nose and rear tail available for freighters, which Boeing has already done on their Dreamlifter
- engine nozzle thrust vectoring, used mainly for takeoffs

inb4 someone posts that ancient 797 image, I'm thinking of hybrid aircraft that are more traditionally shaped and less flying doritos