I dont think it would ever happen like that.
Number 1 reason, for a large and well known company to sue a small time game studio or a hobbyist developer for a game they stole would cause a negative backlash on them, hence lost customers and lost income potential.
Number 2 they will avoid legal battles because they are very costly (even to the big guys), because they have to explain to the shareholders why they are wasting money pursuing a lawsuit with something like that.
Number 3 in a lawsuit, development on both sides stops, hence it will take much longer for them to finish a product and put it up for sale .... which will delay income, possible drop in stock prices and lost consumer confidence. Remember, trials can last for years.
At most if you've worked on a idea that they like, they will throw some cash your way own the IP rights, and if you're a company that does good work they would most likely enter talks to absorb you as a subsidiary. Because in the end its cheaper then a lengthy and time wasting court battle.
But you should be aware, anything you post on the internet (like a game idea, or game concept) its not protected under copyright. Only the models, assets, certain aspects such as names are, anything else is open game for anyone. Thats why I wondered why so many put up WIP's because anyone can make very close derivatives of your idea.
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