The phrase "surprise sequel" refers to a movie or book that unexpectedly continues the story of a previous installment. It's called a surprise because it catches the audience off guard as they didn't expect it to happen. Full definition
There are two other games in that photo that also point to a few surprising sequels that are much anticipated. (heavy.com)
High - end specs and a low entry price were a killer combination for one of the most talked about Android handsets of last year, and the company behind it is already teasing at CES 2015 a still - surprising sequel for this year. (bgr.com)
The Last of Us Part II, Marvel's Spider - Man and God of War all strutted their stuff with fresh footage, too, while both Spelunky and Guacamelee scored surprise sequel announcements we're extremely excited about. (finder.com.au)