If you build it, they will come.
Honestly, I don't think it's that hard to build a world that other people want to explore. All you really have to do is make one you want to explore, then write whatever stories you want to about it. Unless your universe consists of a single mind and only lasts long enough for it to think "Oh, not this again", or some similarly small setting, I don't think you'll ever be able to talk about everything that happens. However, there are a few things that I have noticed really helps things out:
1- Be vague. Like, really vague. If you can condense a two-page story into a one-sentence summary, you've probably given someone enough room to write an entire novel, or more. Just look at the line "You fought in the Clone Wars?" from Star Wars: to my knowledge, it inspired at least two movies, two animated series, at least four video games, and I'm sure countless books. And that's not even counting all the fan theories that must have been flying around before the prequels came out. Many of the fanfiction I write works well here, where there are gaps in the story begging to be filled. This goes hand-in-hand with my second point,
2- Overshare. The Clone Wars had no importance to the plot of that first Star Wars movie; they were over, and because of rule #1 no one knew what effects they'd had on the galaxy. But like I said, it was something to jump off from. This is pretty good advice for any world: don't use everything that you mention. If you use everything up, not only is your world going to appear small and lifeless, but there'll be no room for anything else to happen. This also works with my third point,
3- Leave loose ends. Do you know how aggravating it was when Halo canon stated that SPARTAN-IIs only wore powered armor the last year of the war? And how equally aggravating that the author who introduced the SPARTAN-IIIs also killed all of them off? Both of these facts were stretched to their breaking points in other stories, because everybody wanted to use the cool stuff. Thus, you should avoid wherever possible the bookends of time on your most interesting elements. If there is a cool war, either don't describe how it started/ended, or give it a good century or two in between so people can write stories about its battles. If there is a great weapon/machine, make sure your heroes don't have the only one (or if they do, make sure there are equally cool knockoffs available). Many stories deal with the beginning or end of something, but you should avoid whenever possible doing both.
4- Now, this one is more of a personal belief, and that is don't worry about it. The last thing you want to do is compromise the heart and soul of your work of art by trying to give other people a way to tack onto it. Stories should feel to their creator as if they already exist (see Leaf by Niggle); if you're not exploring this world as much as your readers, and are simply cobbling it together as a means to an end, it's going to show, and it's not going to be good. If you create a great story in a world that you love for itself, rather than for its capabilities, then other people are probably going to love it too. After that, if there are more stories to tell, they should come naturally.