The big one of endgame is Tony Stark commits the single worst crime in the history of the universe by "un-snapping" the half of the universe that was snapped by Thanos 5 years after they disappeared. No accounting for all the people who are dead in the meantime, no accounting for all the awkward conversations of "oh for me I just blinked and now you have two kids and a new wife" or "who are you people and why are you in my house where am I supposed to live now" all because he lacks the imagination to make the universe go back 5 years while keeping Pepper and his daughter the same.

Oh and

Vision and Gamora both die but Gamora gets replaced with a time clone. The Hulk and Banner integrate and become Smart Hulk. Captain America wields Mjolnir and Thor forges an axe called Stormbreaker whose handle is Groot's arm.

You asked for spoilers so I'm not tagging them. Iron man dies, Captain America retires, black widow dies, antman joins in. There's some wonky parallel world time travel hijinks. Think those are all the big ones.