Some designers put an enormous amount of work into their game, and this extends beyond the game, into a larger project.
I see designers taking games of all sizes and qualities, and adding "customisability" on top of the entire structure of the game. This customisability allows the creation or alteration of — for example — decks or characters, before the game is played.
If a designer is building the world's best game, then people will naturally want to play it forever, and tinker with its minutiae. This is a behaviour found in enormous games like Magic.
Most players don't actually want to do this — certainly not for a typical game. It also adds components that all customers are forced to buy, and implies a future line of product offerings for the game. Publishers will obviously baulk at this, but this kind of grandiose planning indicates that the designer wouldn't entertain regular publication anyway.
Just let your game be a game. People will play it for a time, and then they'll stop. Don't try to design an entire new hobby.
Do not design an expansion for your game. Very few games get expansions or become series, and you aren't such an amazing designer that you can leave good stuff out of your first game, and still create a top quality game. Even altering the base game, so that an expansion can easily tie in, damages your game. Put your best ideas into the base game, and treat it as though it stands alone. If you've made an expansion, cannibalise it, to improve your base game.
You might part ways with your publisher, and thus no expansion is possible. You might not want to do an expansion. You might want to work on something new. You might have left game design behind entirely.
If you do get to do an expansion, you'll be a better game designer by then, and there will be huge amounts of community feedback. Any pre-existing plans you made will be obsolete.
Even if an expansion does eventuate, and you make it, most people will play your game without it.
No one cares about your story. Your game is the only story there is.
If you're creating something heavily story-driven, like a TV show or book, then creating a whole world for your characters might be a good exercise. Doing it for a board game (and I see it often) is probably detrimental. Yes, it's obviously a colossal waste of time, but it takes your focus off the original game, and it also constrains you.
I recently played a prototype with an extremely bland theme. I asked the designer how attached they were to theme. They told me the theme couldn't really change, because it was part of a bigger "world" that they had all kinds of plans for.
You'll be inclined to create more games inside this world, instead of trying new things. Also, any publisher will instantly throw your "world" into the bin. Even the existence of such a world might lead the publisher to assume that you would insist on them adopting the whole world along with your game.
Also, players might be daunted or confused by this big picture. Am I starting in the right place? Can I just play one part of this larger game?
Magic used to call its annual product "7th Edition" etc. Potential new players would think they'd missed the six previous six sets, and it was probably too late to start. The company later changed their names to the format "Core Set 2018."
A tournament scene is going to arise around the absolute biggest and deepest games. Even Radlands doesn't have a tournament scene (though numerous tournaments have been held.) It's not something you have control over. I've seen designers hosting tournaments of their unpublished games, and imagining a vast tournament scene arising.
I've also seen game prototypes with complex rules, designed for "when people play it in tournaments".
Many people will disagree with me on this one, but I expect it's served me well.
Yes, playtester feedback is great, and playing other games and prototypes is instructive. But, beyond that, I don't interact with anyone. I don't participate in communities, contests, or conventions. (I do participate in the Radlands community, but it doesn't help me in any way.) I don't think these things are necessary, unless you're planning a Kickstarter slog, which you shouldn't be. Board game success could certainly come from knowing the right people, building a network, marketing, or seizing opportunities, but mine came from endless hours of playtesting, introspection, and detailed work, which led to a very good game. Then, I just emailed a publisher, and they published my game.
I have playtesters, who give me feedback, and sometimes advice, but they're helpers, not co-designers. Also, publishers will demand things. Outside that, I don't work with anyone.
If you're learning from others, you're not ahead of them, and therefore you're not that great.
If you're really good, no one should be able to help you.
Technically, you're learning from me right now, but anyone can do that, so you'll need to eventually advance beyond what I can teach you.
You should be on your own. I do very well on the stock market, which is something very few people can do. I have my own insights and philosophy. They didn't come from anyone else, nor do I tell them to people.
If you want to be great, it's not something that anyone else is going to be able to teach you.