Avengers: Infinity War writers, Christopher Markus and Stephen McFeely, have revealed why the idea of having Thor battle a serpent to obtain his Stormbreaker weapon was dropped. Chris Hemsworth's God of Thunder spends much of Avengers: Infinity War alongside Rocket Raccoon and Groot of Guardians of the Galaxy fame on a quest to find a new weapon, following Hela's destruction of Mjolnir in Thor: Ragnarok. Eventually, the trio come across Peter Dinklage's mystical Norse blacksmith, Eitri, and Stormbreaker is brought to life.
In late 2018, Avengers: Infinity War concept art emerged showing Thor and Rocket battling a giant, multi-headed serpent, with Thor wielding a giant ax and Rocket madly blasting with his trademark firearms. At the time, it was speculated that this snake, most likely the Norse beast of legend, Jormungand, was encountered on an alternate path for Thor to obtain Stormbreaker. McFeely wrote in The Art of Avengers: Infinity War that the snake was meant to provide a "test" for the God of Thunder. Ultimately, of course, this idea was scrapped.
Speaking at SDCC 2019 during their Writing Avengers: Endgame panel, Markus and McFeely revealed why they decided to ax the giant serpent angle. The duo confirm that the scene was part of a side-quest for Thor to reach Stormbreaker, but also claim that the idea was deemed too "adventure-related" and "insufficiently character based."
Although pitting Thor against an almighty Norse monster would've undoubtedly made for a fantastic action sequence, the logic behind these reasons is perfectly understandable. The Thor/Groot/Rocket subplot already felt detached from the main story of Avengers: Infinity War and drifting into fantasy territory where brave warriors take down hideous beasts would've undoubtedly felt like an even greater departure from the rest of the film.
The serpent scene may have been visually impressive, but the duo's claim that the angle wasn't rooted in character development also rings true. Thor may have earned the right to hold Stormbreaker by beating the creature, but MCU fans have seen displays of his battle prowess on many occasions previously, and the serpent fight wouldn't have added anything new to Thor's character in Avengers: Infinity War. On the other hand, having Thor take the full brunt of a burning star in a desperate act of self-sacrifice not only showcased the character's physical strength, but demonstrated how desperately he sought revenge against Thanos for slaughtering the last of the Asgardians.
The final version of Thor's Stormbreaker quest also provided a welcome change of pace to Avengers: Infinity War. The Avengers' first round against Thanos wasn't short on fight scenes and delaying Thor's return to action until the final act makes his eventual return to Earth all the more effective. Furthermore, the scene where Groot fashions a handle for Stormbreaker is a huge moment for the teenage tree, and probably wouldn't have taken place in the earlier version of events. All things considered, it seems Markus and McFeely made the correct choice.