Borderlands dev Gearbox was at one point considered for Halo 4
Before developer 343 Industries was a thing, Microsoft considered letting Gearbox Software develop the next Halo game.
Bonnie Ross, head of Halo custodian 343 Industries, looked back on those days where the future of Halo was uncertain. In a new interview, Ross recalled the time when Bungie decided to go independent, and Microsoft needed to consider all options when it came to Halo.
Speaking with IGN, Ross said that one possibility would have been to end the series. After all, Master Chief's arc sort of concludes with Halo 3, and series fans would have two more games to look forward to, those being ODST, and Reach.
Another option Microsoft considered was possibly entrusting an external studio with making new Halo games, but only one or two. Gearbox Software, maker of Borderlands, was considered. The two had worked together in the past on the PC port of Halo: Combat Evolved, so they were familiar with each other.
This is when Ross decided to offer an alternative solution: establish a new internal team and task it with writing the future of Halo. So, she pitched her idea of where Halo should go next to the higher-ups, and was then assigned a team, which later became 343 Industries.
343 went on to work on Halo Reach before the studio created its first full game: Halo 4. The team also developed Halo 5, and is currently working on Halo Infinite.