For a game as big as Red Dead Redemption 2, it is not unusual to have plenty of content left out of the game during development. Trial and error is part of the process of creating a game, especially a process that leads to a masterpiece such as Red Dead Redemption 2.

Kush Qrox, a Red Dead Redemption 2 content creator, shared an unused cutscene found deep within the PC version of the game file involving the legendary fish quest. The potential quest would have involved Arthur traveling to the areas not possible with him otherwise. The areas include Blackwater and New Austin.
We've all wanted to play Arthur until the end and would have loved to explore the rest of the areas with Arthur. Alas, that was not possible. But, according to the cut content, we were close to being able to do so.
According to Qrox, players could complete the quest given by Jeremy Gill with Arthur. Jeremy Gill tasks the players with catching all legendary fish in Red Dead Redemption 2. Jeremy Gill is also found in the last part of the quest, where he assists the player to the last legendary fish found in Rio Bravo in New Austin.
In the game's final version, players cannot complete the quest and go to New Austin until the epilogue. This is because Arthur cannot move to New Austin as he is a wanted man. Anytime a player tries to enter New Austin, they are hunted by bounty hunters or get shot by an invisible sniper.
Arthur is seen in the final cutscene with Jeremy Gill in the unused cutscene. The location of the final cutscene is far into New Austin, suggesting we would have a very different Jeremy Gill quest if it weren't for the storyline taking the narrative route it took. Instead of John, we would have Arthur exploring the wild west.
Furthermore, according to a comment on Qrox's video, numerous other pieces of evidence suggest the game initially planned to allow players to explore New Austin and Blackwater with Arthur, including the unused dialogues from the NPCs inside the saloon at Tumblewood. Players have managed to use mods to enter New Austin with Arthur, allowing them to interact with NPCs. The dialogues and the conversation between the NPCs and Arthur are unique to Arthur only.
In another piece of news, there are rumors about a potential Red Dead Redemption remaster. Even the voice behind Johns Marston, Rob Wiethoff, is ready to voice the character. The rumors have been going on for quite some time, but unfortunately, there is no official confirmation about the development.