Latest chapter makes me think that the “certain pirate” Shanks wanted to talk about is Kaido.
When Whitebeard began making his move towards Marineford to save Ace, Kaido attempted to take advantage of the situation to attack the Whitebeard Pirates.
To the horror of the Marines, reports came in that Shanks had intercepted Kaido, creating a dispute between two of the Four Emperors. The two captains ended their confrontation with a stalemate.
It was probably at this point that Shanks realized Kaido’s appetite for destruction and he decided to warn the Five Elders that escalating a war with Kaido would only bring casualties in an unprecedented scale which is exactly what Kaido’s after.
Chapter 907 itself revolves around the Rocks Pirates and the debt Kaido owed to Big Mom, so I wouldn’t be surprised if Shanks knew a few things about this monster and wanted to give a heads up to the World Government.