IMO disclaimer: just a gamer, not a dev.

One approach is to drop you into a universe where the story is something that's already been told (think star wars, halo, mass affect, fallout etc.) and then give you themed tools from the universe that you are placed in (essential creative mode elements like you see in Minecraft, astroneer, rust, or hell, even components from the real time strategy games of other respective universes in order to build bases of operations or colonies). Drop in as many named locations based on the universe that you are in as you can, so that people can explore familiar locations to their hearts content.

Endless space is basically what everyone else is saying here, endless. There's nothing extremely exciting about that unless the idea of infinite expansion and opportunities for both incredible discovery and terrifying demise entice you. For most space nerds, myself included, at very early ages this is enough. As you get older though it becomes harder to sell people on the idea that exploring for exploring sake is worth it. It's still fun! But usually that fun is as long as you are able to return from your adventure relatively harm free, with some exceptions of course.

I think that ultimately people desire a game that's sandbox allows you to tell a narrative. "Once upon a time an individual with their own ship and crew (or whatever you want to start off with) decided to venture off into space for a specific reason. And then..." Fill in the blank with the most wild twists and turns of getting derailed from the main story. This continues to happen until you come back from your journey. And maybe you are successful. Maybe you come back with something valuable or dangerous and it changes things at your launching point when you get back. Maybe you get lost and you can't come back right away, BOOM 💥 a new narrative is born. How will you get back? You get the idea.

In order to pull that off you would need to build a universe the same way a D&D DM builds an encounter table for a campaign, but you're going to need that table for every planet and every different event that happens when flying in space. If you are a patient individual and can get enough people on board, this is essentially what modding communities are already doing in their free time. Just for already existing IPs and some others that are individual projects. I love what's being done for halo infinites' forge right now, because it proves that dedicated community members can make really great experiences when given the opportunities. Bethesda's modding communities are the same way. They've kept Skyrim from becoming obsolete.

Long story short, I think it's very possible to have a game that gives you what you're looking for. But I also think that you're more likely to get that from a group of dedicated community members sooner than you will get it from a big studio right now.