First ... I am 100% guessing but there are enough clues out there that I think I've put the puzzle together (and I'm probably not the only one).
#1) Lil Nas X posted a picture with Rihanna to Twitter on Wednesday tagging "Fenty Skin" which is Rihanna's new skincare line. At the time it looked like a promotional image which is pretty normal and she wanted both of them to be in that particular promotional/endorsement image for whatever campaign it would be used for. It makes sense.
#2) Here's where I started to become suspicious. Lil Nas X then tweeted a video where he was apparently chatting with Rihanna and she was commenting about hearing a clip of music Lil Nas X was working on. No big deal right? Obviously anyone would freak out over Rihanna complimenting their music.
#3) On Thursday evening Columbia Records tweeted out that same picture of Rihanna and Lil Nas X but with no content. Lil Nas X is signed to Columbia Records. I could be completely wrong but I don't see why they would be doing this in relation to the Fenty Skin deal. If so, they would put some type of text congratulating him, etc. Instead they made the post super mysterious. This makes me think they're dropping a subtle hint that a song from the two of them is coming. We know both are working on music. Both have been talking about it on social lately.