Shin Nihon Kikaku, literally translated as new New Japan Project.
It's worth mentioning that the Chinese company (Leyou Millennium) that bought out SNK back in 2015 has been pushing to get SNK ip in crossovers in order to expand marketing. And they've had quite a few since then. Geese in Tekken, Terry in Fighting EX Layer, Haohmaru in Soul Calibur, Iori in Square's Million Arthur fighting game, and samsho characters in Granblue fantasy. Smash x SNK is surprising, but makes more sense in context.
That being said if there's an SNK rep, I would bet Terry, Mai, Nakoruru, Haohmaru, or Athena. (either classic goddess or Athena Asamiya) Kyo and Iori are also possible picks.
Personally I want Mai, but it seems strange to include a character that shows so much T&A after characters like ZSS and Snake got such a hard booty nerf :' (
or maybe we'll get Kula 😛