Make them about submarines instead of spaceships.Edit:
It would have been kinda funny to leave it at that, but I feel the need to expand and explain my statement.
First of, what do i mean by "slow space game", here I'm simply referring to games with spaceships speed-capped at ridiculously low speed. Examples I know of being Starmade at 50km/h and Space Engineer at about 110m/s.
And why "make them about submarines instead of spaceships"? That thought came after seeing some gameplay of Angels Fall First
. It had this one asteroid map so cluttered and blue it looked like they were dishing it out between some glaciers under the north pole. This made me think "If they kept everything, kept the bigger ships upright, put a floor and had fishes swim about this would make an awesome sci-fi submarine battle game". Who said submarine couldn't shoot lasers and deploy aqua-fighters?
In addition, If you do not exploit any trademark space physics (speed, range, newtonian physics ...etc.) setting your game under the sea will give so many more interactive landscape possibilities. For example, what do you expect to see as terrain in space? Some asteroids, maybe a station, a planet in the background perhaps. Underwater you have fault lines, mountains, coral reefs, abysses, kelp forests, iceberg clusters, active volcano chains and so on.
Much more could be said but I like keeping things short, let me know what you think.