One of my friends told me that in Guardians of the Galaxy, you could see War Machine and Nebula in 2014 on Morag, when Star-Lord was dancing. I haven't gone back to rewatch GotG 1 yet, but I wondered if anyone could confirm this. Also, are there any other instances in previous MCU movies that the 2023 versions of the Avengers can be seen?
There is an interview with the Russo Brothers: https://bgr.com/2019/04/30/avengers-endgame-ending-russo-brothers-on-captain-america-iron-man/
where they talk about this:
Q: [Endgame’s] plot, is it a parallel universe or a closed time loop?
A: Nope, not a time loop. Both Ancient One and Hulk were right. You can’t change the future by simply going back to past. But it’s possible to create a different alternate future. It’s not butterfly effect. Every decision you made in the past could potentially create a new timeline. For example, the old Cap at the end movie, he lived his married life in a different universe from the main one. He had to make another jump back to the main universe at the end to give the shield to Sam.
I'd like to stress on the line:
Every decision you made in the past could potentially create a new timeline.
... not just taking of the Infinity Stone
When Rhodes and Nebula went back to 2014, to Morag, the event them being there causes the two Nebulas to interlink. This event creates a new timeline. This timeline is different from the Original Timeline where the events of GOTG vol.1 takes place. The Original timeline does not have the future Rhodes or Nebula in Morag in 2014. Also, there is no such scene in the GOTG film. Your friend is messing with you :)
To add. the timeline that was created because of the Nebulas will continue to exist even after Steve returns the Power Stone because taking of the Power Stone is not what caused that timeline in the first place. In this timeline you will not have Thanos because he went to the Original Timeline to 2023 and died. So the movie GOTG vol 1 would not play out the way it did without Thanos.
For in-universe, the answer should be NO
All the movies before Avengers: Endgame follow the Universal Timeline. In Avengers: Endgame, when the Avengers go back in time, they create branches in the the Universal Timeline. All this branches will have new realities (and movies) now but thankfully, all of them are clipped at the end of the movie, when the stones are returned.
So, in the Universal Timeline, in which earlier movies exist, there is no possibility of such a thing happening.
For out-of-universe, there may be a possibility of such Easter eggs. But I doubt it. The entire MCU path wasn't planned from the start. Also different directors made different movies.