there’s one very specific situation in deathsmiles in which killing the penultimate boss without bombing it (but losing lives is ok) gives you a cake that sets your lives to max for the final fight
Ikaruga feels like an iteration on this idea. Introducing Z plane movement would add new ways of avoiding attacks, possibly allowing one to sidestep whole waves. The problem is, how do you make that legible and predictable? With Ikaruga, the team went for extreme legibility through a limited palette. Even though you don’t move between planes, there’s still something there about moving freely and having more options to avoid attacks.
If you are thinking about that I feel like 3D Ps1 (raycrisis, einhander, rtype delta) do this as does my eternal champion Astebreed. Lots of movement and camera changes.
something like omega boost except with a fixed camera (i.e. doesn’t move along with the ship) and traditional shmup design where the stage is constantly scrolling at you PSX Longplay [641] Omega Boost (US) - YouTube
or something like zaxxon if you could move forward and back along the stage
or like vertical force if the z axis was fully usable and not just two lanes
basically, you have a ‘shoebox’ of a 3d space, you fill that 3d space with bullets and a scrolling stage, you put a ship in there somewhere. the challenges are how to make the game legible (camera location, silhouetting/transparency etc) and balanced (bullet vectors, power, etc)