#1 Heel: Alexa Bliss

Alexa Bliss and Nikki Cross have been close friends and tag team partners for a while now. Many people were expecting Bliss to turn on Cross at the beginning of their alliance but Little Miss Bliss proved her doubters wrong. That isn't to say that the team won't split apart someday though. With SummerSlam approaching, WWE could create an interesting rivalry between Bliss and Cross.
Bliss could blame Nikki for the fact that they lost the WWE Women's Tag Team Championship to Bayley and Sasha Banks. She could even attack her tag team partner for having a SmackDown Women's Championship match against Bayley at WWE Extreme Rules.
Alexa Bliss was always better as a heel, so turning on her own best friend is the only sensible way for her to undergo a change in persona. They are good as a tag team, but they could do so much on their own.
Bliss could go back to chasing the top titles while Cross could also go on to have a singles career and become a star on her own.