During an interview with Game Informer for its May 2017 issue (number 298), a few of Horizon: Zero Dawn's developers from Guerrilla Games discussed some potential elements that could come about in the open world action-adventure title's sequel, with Narrative Director John Gonzalez hinting at the possibility of the original game's protagonist Aloy starring in the immediate followup. Although the Narrative Director didn't explicitly state as to whether or not Aloy would be the main character yet again, his statement definitely hints at her inclusion in the sequel as the lead role.
When Gonzalez was asked about what would be important in the sequel for Horizon: Zero Dawn, he explained that one of the major priorities would be to respect the "emotional connection" fans have with Aloy, and attempt to build a game that "resonates just as deeply." Of course, considering the success of the new IP for Guerrilla Games and Sony, the development team obviously doesn't want to alienate fans by not doing the initial installment in the franchise justice, as Gonzalez acknowledges the importance of creating the followup correctly, and continuing Aloy's story would definitely be a step in the right direction.
"We’ve been absolutely humbled by the players’ response to the story and their emotional connection to Aloy. Should a sequel happen, our number one goal would be to respect that connection by doing our utmost to create a story that resonates just as deeply.
"We feel duty-bound to honor the time that players have invested into Horizon: Zero Dawn. The last thing we want to do is disappoint the folks who have given the game such a passionate support."
Of course, it's quite possible that Aloy might not be the central focus of the immediate followup to Horizon: Zero Dawn, as Guerrilla Games developers have previously teased the possibility of sequels featuring different protagonists. However, it would be anyone's guess as to who would be the main character in another entry, as the title is chock full of vibrant and interesting figures that could anchor a story on their own.
That said, though, just prior to Horizon: Zero Dawn's release, Sony Interactive Entertainment was already confident in the new title having plenty of staying power, as the company announced high expectations for franchise potential with Aloy being "a PlayStation icon of the future." With this being the case, it's highly likely that Guerrilla Games will have the female lead remain as the series protagonist for now, particularly when it comes to an immediate sequel.
Taking all of this into consideration, if Sony and Guerrilla Games wish to maintain a coherent connection with Horizon: Zero Dawn's fan base, it would make little sense to switch out Aloy for another protagonist in the next entry in the series. After all, fans already seem to have an incredibly potent connection with her, especially since fans were excited to cosplay as her without even having played the game first.
Horizon: Zero Dawn is available now for PlayStation 4.