I've done the 1st sketch for poses, and some color ideas for the backgrounds too.
I've tried to do some complementary poses for the characters, in a somewhat ready to fight pose.
As for the backgrounds, the Shinobi background features blue & purple, and I can add some red too, also, the character wears red. And I thought of adding the Master System grid to it, fitting both the original cover and the new retro aesthetics.
As for Ninja Gaiden, I used red, yellows and oranges, as in the original Nintendo cover, with the city skyline too, and I thought of adding the fire and smoke like in the original cover.
Both color palettes are perfect for new retro, one for night and the other for sunset, and have references to the original covers.