Copyright

No one is going to steal your game or game idea.

Ideas are Worthless

Your idea has no value. The value is in the execution.

No one is interested in your half-made game, and until very late in development, your game isn't publishable anyway. It might be a 6.5/10 game up until the last 10% of its development time, before finishing as an 8/10 game. Even if someone stole your game in its entirety, they'd have to keep working on it.

People think their own ideas are brilliant, and that's what they want to work on — not your silly game.

Theft of prototypes is basically non-existent. Knock-offs of successful, published games, are far more common.

A publisher who stole an amateur designer's game would instantly be ruined by the bad publicity.

Work with other people on your game, all the way through. Start with people you know well, and build up to playing with random gamers, via the internet. You'll feel much more comfortable with this, once you get to know a few people. I don't just post my game for anyone to download and play, but a few major designers actually do this.

What you gain from other people's input is crucial. Even if there was a real chance of my games being stolen this way, I'd still do it. There is almost no way to develop a great game in secret.

Legal malarkey

Don't get a patent. A patent or NDA (non-disclosure agreement) marks you out as a laughable newbie, and publishers will likely reject your game on this information alone. You also can't patent ideas; only specific text, pictures, and explicit concepts.

As such, you can legally make a game very similar to an existing game. Your "Scrabble but with numbers" game is legal, though most publishers aren't going to be interested in it.

It's probably legal to take an established game, put a new theme on it, but leave all the rules and components almost exactly the same, and sell it. However, that kind of thing rightly receives flak, and I've never seen it be successful.

Almost all games are similar to, or evolutions of, previous games. Just make sure to add at least one thing that's new or different, and no one will complain or send in the lawyers.

Don't use an existing IP

You are not going to be able to get the rights to My Little Pony or Star Wars. Those companies will deal with established professionals if and when they want a game. If you really like an existing brand, just create your own fantasy world, that's clearly inspired by that world. You also don't have to pay anyone royalties.

"Dinosaur Island" is clearly a Jurassic Park-themed board game, even though it doesn't say so.