I'd almost turn everybody heel lol. Ambrose, Roode, Nia Jax, Sasha Banks, Chris Jericho (should never be a face anyways), Tye Dillinger but the 2 who'd benefit the most would be Balor and Reigns.
They subtly gave it a go with Finn for a while when he reunited with Gallows and Anderson and I thought this was going to be his break out season tbh but then they went right back to solo babyface Balor and I also doubt it will change much given that he's kinda like their LGBTQ ambassador now.
And Reigns is an obvious choice. Just like the rest of the Anoa'i family, he's a natural heel and it would totally help his character work. He's already proven that he's a great worker, has hustle for the business and the people boo him already anyways, so why not just go with that. The company was in this situation before with people like the Rock, Orton, New Day, Charlotte, Batista, so on and so on and all they did was turn them heel, and people started liking them again