These are the sketches I've made for my metroid styled doorway.

Before these I made a few other sketches of things that I didn't carry forward into this design.

 An origami like folding door would have been interesting to animate, but it just looked far too complicated and didn't keep in with the metroid style I had commited to.

 The bottom image is of various door designs, I kept trying to think of different ways a door could function.
Bottom left - shutter like design
Bottom middle - swivelling panels that also shift outwards
Right - a magnetic sort of spinning door in the style of these toys

 Top - opening shell, dome like for missile resistance.