Avengers: Infinity War saw the defeat of Hulk, wherein he took a beating from Thanos before he was sent back to Earth by Heimdall through the Bifrost.
While fans were anticipating a rematch between the two in Avengers: Endgame, unfortunately, that never happened in the film.
Instead of that, Hulk merged with Banner and became Smart Hulk who went on to undo the effects of the snap that Thanos had made using the Infinity Gauntlet.
Production Ideas and Alternate Scenarios:
Regardless of everything that actually happened in the film, the makers, along with the writers of Endgame, had at some point, toyed over the idea of a rematch between Hulk and Thanos.
The D23 Expo by Disney was a place where many new announcements and details regarding the Marvel Cinematic Universe were unveiled and revealed.
One of these included the concept of art for the scene ideas of the rematch, which were shown at the Expo.
This artwork, along with other similar ones for various other scenes, showed Iron Man, War Machine, Spider-Man and Rescue flying into action. Others showed a kiss between Tony Stark and Pepper Potts on the battlefield.
Opinions of The Writers:
Stephen McFeely and Christopher Markus, the co-writers of the film, even went on to explain the reasons why the rematch did not take place.
They commented that while the battle had a lot of things going on, it wasn’t as long as fans might assume it was. And while fans have been waiting for a rematch between Thanos and the Hulk, it would have made much more sense if he had not transformed to Smart Hulk.
After this transformation, the Hulk is a completely different character, who is not driven by the hatred he once used to be.
Additionally, at the San Diego Comic-Con 2019, the writers also said that they initially had plans to introduce the new Smart Hulk towards the end of Avengers: Infinity War, but they did not go ahead with it because they felt that its tone was in contrast to the moment going on in the movie.
And by then, they had already shot Endgame, wherein he already had become Smart Hulk.