The only two real possibilities are Luffy or BB. Shanks wasn’t going to the Gorosei to snitch about the silver medalists Kaido and BM, and he certainly wasn’t snitching about Kid, Law, or any of the bronze medalist Warlords. BB is the obvious choice since Shanks was already shown to be wary of him, but BB is such an obvious choice that it doesn’t even make sense for Oda to be so secretive about who the “certain pirate” is. The end of Wano showed that Shanks clearly has some interest in Nika, so it wouldn’t be surprising if he was actually snitching about Luffy. After hearing about how Luffy defeated YC1 Katakuri and was declared the “5th Emperor”, Shanks probably realized that Luffy would soon awaken his DF and decided to inform the Gorosei about it.
Given Shank's whole "keeping the peace" mentality, I wouldn't be surprised if it's our local Sun God at this point in the story. You are right; it is quite predictable for it to be Teach. He's already an
existential threat to Shanks, from the moment he underestimated him when they were kids to the fateful duel years later. In the Oden flashback, Buggy correctly stated that he is a monster. Shanks just thought the guy was having twice as much fun from lack of sleep. To add more salt to the wound, there was no way Shanks could attack him when he was still a crewmember under WB.
Meanwhile, Luffy out here doing everything and being put in constant danger during his journey, and Shanks has been protecting him through some direct interventions like Marineford. Not too little, not too much, just a small
nudge to make sure he doesn't die. I like to think he gave the Gorosei a sinister warning or an ultimatum.
At the end of the day, there are no two characters who live rent-free in Shanks head more than Goofy and Mr. Zehahahaha.