The references could indicate that Konami is working on Master Collection Vol. 2 with the other mainline games in the series.

Metal Gear Solid: Master Collection Vol. 1 is finally available worldwide, allowing many to relive the first three mainline games with Solid Snake and friends. However, as suspected when the first volume was announced, Konami is seemingly planning a Vol. 2 with the rest of the mainline series included.

This is due to Timo653 on Reddit discovering multiple strings related to Metal Gear Solid 4: Guns of the Patriots, Metal Gear Solid 5 and Metal Gear Solid: Peace Walker when looking at scripts for the launcher of Metal Gear Solid 2 in Vol. 1. There are references to manuals, Terms of Service and more, with Peace Walker having more than Metal Gear Solid 4 and 5.

Vol. 1’s titles are also included in the strings, lending further credence to Vol. 2 being in the works. Konami hasn’t confirmed the latter, so we must wait for an official announcement.