
I'm working on a new game that reuses a lot of Souvenir's code, so lately I've been doing more research into non-monoplanar first person movement, meaning you're not limited to primarily moving along the X and Z axis / traversing across a single, fixed, designated "ground" plane.
Traditionally, "noclip" flying / spectator modes have been the most common form of non-planar first person movement. However, I'm not a fan of it as a movement mechanic because you're always "right side up" above a ground plane. Your idea of space never really changes because that's not the point.

Shattered Horizon (2009) was a multiplayer FPS in space, where astronauts shoot each other while hovering around asteroids. From what I can grok in gameplay videos, players can rotate and hover freely, but they almost always maintain the same common "ground plane" as if there's a "right side up" in space. Common map layout terminology and directional lighting also reinforce the idea of a "top" of the map.