In the second half of season 10 of The Walking Dead, Beta (Ryan Hurst) could replace Alpha (Samantha Morton) as the leader of the Whisperers. Alpha has the been the main villain since season 9, but considering that the series began laying the groundwork early into season 10, it seems plausible that Beta could end up being the biggest threat to the main characters.
To win the war, Alpha is playing the long game by gradually whittling away at her enemies. Beta has stood behind her the entire time, but the resolve of another key character, Gamma (Thora Birch), appears to be shaken. The Whisperers' plan hasn't gone without hiccups, but as of The Walking Dead's midseason 10 finale, they still seem to hold the advantage. It was revealed that Alpha had a surprise up her sleeve when the survivors stumbled upon a cave that housed Alpha's zombie horde.
So far, Alpha's plan to defeat the united communities has been the primary focus of season 10, but it's possible that Alpha could be ousted or killed prior to the conclusion of the Whisperer War, which is likely to take place in the season finale. If this were to happen, Beta could succeed her as the new Alpha and serve as the show's main antagonist for the remainder of the season.
Alpha being defeated early doesn't seem like much of a stretch. Before the season had even reached the halfway mark, the show had already exposed Alpha's biggest weakness: Lydia (Cassady McClincy). Despite everything that Alpha has preached about loyalty to the Whisperers and casting aside the things that make them weak, Alpha violated her own principles by allowing Lydia to live. If Alpha's hypocrisy is exposed, she could lose the confidence of her followers. This would force Beta to take command.
Beta could end up being a much more competent leader of the Whisperers for two reasons, the first of which being the fact that he, unlike Alpha, doesn't trust Negan (Jeffrey Dean Morgan). His distrust of Negan will keep him on his toes, whereas Alpha is considerably less suspicious of their newest Whisperer (who actually does kill her in the comics). This mistake could open her up to treachery. Also, Beta doesn't share the weakness that holds Alpha back. Beta has completely bought into the Whisperer philosophy - even more so than Alpha herself. Beta perfectly represents everything that the Whisperers believe in, making him a suitable candidate for leader, and perhaps an even more dangerous enemy for Alexandria and the Hilltop than Alpha.