assign a space or empty dimension to be a "galaxy"
create structures to represent "planets", and set up teleportation to/from planets and the galaxy
set up various gravity effects on planets, and implement spacesuits/armor that can negate those effects when worn
add additional planets through datapacks
light speed travel system for traveling between galaxies, especially for use in modpacks with multiple space mods (for whenever it is I write those other mods…)