To start with, I'd cut out the following:
Hawkeye completely, except to give him a couple minutes of screen time where he can be the one to jump off the cliff and die instead of Black Widow. Cut Ronin Hawkeye. Cut Hawkeye watching his family turn to dust, since we already saw everyone turn to dust in the prior movie.
All the back and forth trying to convince Tony Stark to help them and him refusing.
Smart hulk in the cafe signing autographs.
Smart hulk trying to discover time travel and turning Ant Man into a child/old man/teenager.
Smart hulk handing out tacos.
Cut most of fat Thor out. Maybe just leave him as lean Thor, and remove most of the cheesy comedy.
Cut out most of the ending, the funeral, and old man Captain America.