Some Red Velvet fan did some detective work using just a teaser that had silhouettes of the artists just a week or so ago and figured out that it was going to be John and Wendy, and it was recently revealed/confirmed to be true. Song comes out in a little over a week. Here's an article from Vogue on it.

https://www.vogue.com/article/john-legend-red-velvet-wendy-duet-written-in-the-stars-behind-the-scenes-diary