The end of Avengers: Age of Ultron was actually changed due to a pretty big internet scoop that turned out to not be true in the end. As it turns out, the spoiler did come from a place of truth, because this particular spoiler was actually on the drawing board for the MCU at a particular time. Age of Ultron ends with the Hulk defeating Ultron and taking off in the Quinjet in a quest for solitude, last seen flying over the ocean and the team loses signal from him somewhere "over the Caspian Sea."
Marvel head honcho Kevin Feige has recently revealed that Avengers: Age of Ultron wasn't actually going to end that way and that we were going to get a clearer picture of where Bruce Banner was heading. In Thor: Ragnarok, it is revealed that Hulk has been living in Sakaar as a famous gladiator and he got there in the Quinjet after falling into a wormhole, but Feige revealed that Age of Ultron was going to show Banner disappearing into space in a way that was more pronounced than the vague way that we were left hanging at the end of Age of Ultron.
While talking with The Wrap, Kevin Feige revealed that a popular rumor/spoiler dictated the end of Avengers: Age of Ultron. The ending was to show the Quinjet taking off into space, but the spoiler of the Planet Hulk project threw a monkey wrench into the whole plane. Feige explains.
"You had the big scoop about us doing Planet Hulk. It turned out to be accurate (but) at the time, it was not accurate because we didn't have any plans to do it and Joss (Whedon) was doing Age of Ultron at that time. The way it ends as you recall with Hulk in the Quinjet going off, right? We said, 'Joss, we can't do that because he's not going into space and people are going to think we're doing Planet Hulk because Umberto has some big hot scoop that we're doing Planet Hulk, we can't do it.' He goes, 'Well, what are you going to do?' We said, 'We don't know.' So, if you go back and look at that shot of the movie, he's in the sky. It's blue sky."
The spoiler actually made an impact on the ending of Age of Ultron in an attempt not to mislead fans into thinking that the Planet Hulk project was green lit and officially happening. It's interesting to see that Kevin Feige and the rest of the MCU decided to slightly change the ending of a project to keep away from misleading the fans.
The rumor that Kevin Feige mentioned first came on the scene in March of 2013, which is around the time that Avengers: Age of Ultron was being put together and written by Joss Whedon. Originally, there was going to be reference of the Hulk being detected near Saturn, but it was to avoid too much suggestion of the Planet Hulk movie. That's why Black Widow last gets the signal disappearing over the Caspian Sea instead of Saturn in the final cut of the Age of Ultron.
As to why the MCU didn't follow through with the Planet Hulk movie, Kevin Feige says that they wanted to do something different with Thor. As previously reported, Chris Hemsworth was getting a little bored with the Thor franchise and wanted to get some other Marvel characters involved in the trilogy, so they decided to do something big. And really, what's bigger than the Hulk? You can check out more of Kevin Feige's thoughts on Thor: Ragnarok and the Hulk's involvement via The Wrap.