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In a rough cut of a scene that was eventually removed from Infinity War, Banner can be seen arguing with the Hulk while operating the metal "Hulkbuster" suit, much as he did in the version of the movie that audiences eventually saw, but with one key difference. While in the final edit, the Hulk refuses to help, forcing Banner to battle Cull Obsidian in Wakanda by himself, the alternate scene involves Banner telling the Hulk: "I have an idea..."
This scene would have led to the on-screen origin of Professor Hulk, the character that we later see in Endgame which brings together the size and strength of Hulk with the personality and intellect of Banner.
"We wrote it, we shot it, and went on to shoot most of Endgame, and he achieved union with the Hulk while inside the Hulkbuster," said Christopher Markus, co-writer of both movies. "It didn't work," he added. "We had already shot Endgame, where he was already smart Hulk."
The Russo brothers decided that there was already too much going on in that climactic final act of Infinity War to warrant the introduction of a new character (or at least a new version of a character), and so they postponed that particular development until the next movie.
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This led to Christopher Markus and Stephen McFeely writing and filming the diner scene in Endgame during reshoots, in which Professor Hulk explains to Ant-Man, Black Widow and Captain America how Banner and Hulk fused together through gamma radiation at some point following the Snap.
This makes a minimum of three different endings for Banner in Infinity War. In addition to the pulled Professor Hulk scene, Ruffalo has previously stated that the Russo brothers shot one conclusion to the film in which his character died in the Snap, and another where he survived presumably to prevent him from accidentally spilling any spoilers.