Games programmed the way it is. That just wouldn't work out. Game is balanced around current 9 monsters.
Paul would have to make new abilities. Y, M, C, A. Hits in a Y pattern, C pattern, etc to make up for 16 monsters and it would just change the whole game setup way too much.
He could however, add another boss type monster or new quest, or 6th slot aka observer slot so other people can pop in to chat or watch how group climbs go. That would be fun new content.