Avengers: Infinity War concept art reveals that the Hulk was originally set to fight Cull Obsidian.
In Avengers: Infinity War, Bruce Banner’s storyline involved the Hulk becoming disillusioned with his puny alter-ego. After all, up until that point, all Banner had done in the Hulk’s eyes was simply cower and wait for his unappreciated alternate form to save his ungrateful butt. In the finished film, Banner uses the Hulkbuster to prove that he can be a hero without the Hulk’s assistance by killing Cull Obsidian, before hinting that he and the Green Goliath had to have a long talk after the battle (which foreshadows the Professor Hulk persona in Avengers: Endgame).
But this wasn’t always the case. Up until fairly late into development of Avengers: Infinity War, the Hulk and Bruce Banner were going to put their differences aside to emerge from the Hulkbuster, giving rise to the Professor Hulk persona much earlier into the two-part story. However, it was decided that Banner was in need of character development independent of the Hulk, so he instead got a heroic moment of his own. Check out the concept art of the Hulk’s big, unused moment in Avengers: Infinity War below:
"Hulk vs Cull Obsidian" concept art for Avengers Infinity War by Ryan Meinerding from marvelstudios
The directors stated that having Professor Hulk emerge in the midst of an Avengers: Infinity War scene where it was clear that the heroes were about to lose would have interrupted the flow of the story that they were trying to tell. However, this meant that they would have to explain the transformation in the following film, so the reshoots for Avengers: Endgame added the sequence of the Hulk in the diner. In any case, the Infinity War concept art is a striking image that would have been great to see on the big screen.
The premise of Avengers: Infinity War is as follows:
As the Avengers and their allies have continued to protect the world from threats too large for any one hero to handle, a new danger has emerged from the cosmic shadows: Thanos. A despot of intergalactic infamy, his goal is to collect all six Infinity Stones, artifacts of unimaginable power, and use them to inflict his twisted will on all of reality. Everything the Avengers have fought for has led up to this moment – the fate of Earth and existence itself has never been more uncertain.
Avengers: Infinity War stars Robert Downey Jr., Chris Evans, Chris Hemsworth, Mark Ruffalo, Scarlett Johansson, Jeremy Renner, Elizabeth Olsen, Paul Bettany, Chris Pratt, Zoe Saldana, Dave Bautisa, Bradley Cooper, Vin Diesel, Pom Klementieff, Karen Gillan, Benedict Cumberbatch, Tom Holland, Sebastian Stan, Chadwick Boseman, Paul Rudd, Benedict Wong, Letitia Wright, Danai Gurira, Winston Duke, Tom Hiddleston, Tessa Thompson, Benicio Del Toro, Tom Vaughan-Lawlor, Terry Notary, Carrie Coon, and Josh Brolin. The film was directed by Joe and Anthony Russo, and features a script written by Christopher Markus and Stephen McFeely.
Source: Reddit