Different Shooter Styles

[Arctangent]

essentially become a series of interconnected wall running paths

Only marginally related, but now I kinda want a Quake-style FPS where your "down" is always your nearest wall, so you could be evading shots or attacking from virtually any direction from targets potentially standing on any surface.

Isn't this basically just Super Mario Galaxy in first person?

... Didn't Prey have that as an underutilized gimmick?

[Reactor]

Another neat little trick would be a gravity-jump item. Once you activate it, the ceiling becomes the floor & floor becomes the ceiling. This gravity-shift could be used for a million and two ambitious little things. Puzzle solving, secret areas, making bossfights easier, entering heavily guarded fortresses without having to crawl thru a filthy sewer or a cramped ventilation shaft and get minced by a fan's blades. It's yummy stuff, but you ought to be extremely careful with it at outdoor areas...or else!