IA, all retro missions have beginner, intermediate and advanced loops, like some waves of Chicken Invasion, which are same, but appears on different difficulty. (Spin Class & Spin Cycle)
These loops should be depended on the difficulties of retro missions and in the case, we need some new waves for retro missions with same loops. Please think about it, when you had time. Thank you. 
EDIT: Retro missions may have randomized waves like other mission types and not exactly specified what wave should be next.