I'd add a bunch of small, optional pDLCs that aren't interdependent but could combine.
Several types of additional landable planet type pDLCs, with thicker atmospheres, weather & corrosion that both shapes the terrain (more than just a height map) and affects flying & damage to the ship such that simply flying & landing on them adds some sort of minor challenge in addition to the high-G and rough terrain challenges we currently have.
Optional extra bio-scanning and mining/sample collecting tools to use on existing assets found in space & on planet surfaces to improve mats collecting & recipe unlocking for engineering, crafting & synthesis.
Optional extra region & planet permit unlocks, for example additional Thargoid content within permit locked regions, additional extinct or abandoned alien settlements, POIs & USSs, and a moon permit.
Optional extra 'galaxies' to explore, potentially the whole local group but mainly I have the large & small Magellanic clouds in mind, it would be a shame for a tool as awesome as stellar forge to only be used to create one galaxy.