They had Nikki in a TV taping match before Takeover War Games, so I was under the impression that her Smackdown appearance was a cameo for the time being (like Bayley being a surprise tag partner for Sasha a few months before she was officially added to Raw). I'd wager a beer that she gets called up full time around the Rumble.
Probably no high level NXT feuds for Nikki with Bazsler/Kairi still ongoing, but they could maybe have her put over Candice La Rae before getting called up. Albeit that match probably wouldn't be a great bet to crack the next Takeover, although they could maybe have a fun TV match with Black/Cross vs. Gargano/La Rae at some point.