Tay.
If they aren't going to put her and Sammy together on camera and build their heel personas up by using them together being organic go-away heat, then... there really isn't anyone else that should fit. Serena with the shyts but that ain't what she needs, and Leyla cool and all but she too softly-built for what they're going for. Let Regal talk for Tay, highlight her judo/jiujitsu, and go from there. Perfect opportunity to build her into something instead of just Sammy's also-ran, or the girl who gets stomped by PVZ.
That, or they need to snatch Hayter away from Britt - do that blowup and let Britt go back to being a 2-person act with Rebel. Hayter could fit.
Most important thing for the woman in the stable is to be able to look like they're the type who is with the shyts at any and all times, but also not necessarily have too much of an identity already established from promos. Whoever it is would be better served having promos cut
about them and their asskicking prowess, instead of promos cut
by them about it. Let Regal sell her to the audience - Bryan don't need anyone talking for him and Moxley is... well. Mox.
Wait. No.
Mercedes Martinez. They ain't doing shyt else with her. Let her join the team and actually get some solid wins on screen for a while, even if it's just stomping Joshi. Gotta get her some legitimacy before they use her as a sacrifice to someone else's push later. Means nothing if all she does is lose.