When I am not working on setting up my Classic Dungeon Crawling fantasy sandbox, I am exploring ideas for a Scoundrels with a Space Ship campaign. Stars Without Number and Scum and Villainy are both popular systems for these kinds of campaigns (though Traveller also still gets mentioned, but I don't know anything about it), and while they are on very different ends of the spectrum as game mechanics are concerned, I think they both aim for pretty similar styles of campaigns and adventures. Travel the galaxy and see what happens. I guess most Star Wars games should also work decently well with that premise, but they also very much promote scripted stories as a campaign format.
I think a big thing about open world campaigns in a space setting is that you don't need maps. Even a relatively small section of space has hundreds of stars with thousands of planets, and almost all of them will be of absolutely no interest to the players or any NPCs. The stars themselves don't matter either, it's only a few planets, and these planets are too big to map comprehensively. Travel around a planet is usually by air or the very least by fast cars, and in the few occasions that it is not, the players are most likely trying to get out of the wilderness as fast as possible with no time to roam around and explore. Significant ground travel will usually be in basically a straight line.
The one situation in which maps for space become meaningful is when the ships have limited range and must take stops for refuling if they go for longer distances. In that case it does become relevant for the game to have the players decide which planets and stations they want to stop at, because things might happen while the players refuel their ship or wait for repairs. In a setting where any ship can go from any planet to any planet in one go, like in Star Wars, there is no practical use for space maps at all.
The only maps that really matter in such a campaign a person scale maps used to determine cover and plan tactics in gunfights. For some campaigns, it might actually be useful to have a couple of generic maps prepared for military bases, criminal hideouts, sections of spaceports, warehouses, or bars. Any kind of place where the PCs might run into their enemies.
The question that I have is, how do you set up a sandbox like this?
I guess the big points to consider even more so than in other open world campaigns are developing factions and creating conflicts between them. Creating some influential NPCs in advance might also help to some degree, but I think it would be difficult to determine who really might make appearances during actual play as the campaign is developing. Faction leaders and their preferred people to send out to other planets to deal with trouble hurting their interests would be the most useful. The later are quite likely to come to the PCs, and the former have some chance to have the PCs come to them. Some generic faction members could also come useful at some point, but I wouldn't assign them to any specific location or position. They would make for good quantum ogers that the players could encounter anywhere and whose home is only determined when the players actually meet them.