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I'm not sure I understand your "McDonalds" scenario.
If you do get an idea for a new device and don't want to give it up to the company, you can setup a Trust and assign the idea to the trust as a separate legal entity from yourself. That way, the trust is what owns the idea and then patents the device, and not you who signed the legally-binding agreement.
I ran into a patent attorney who said that there were ways around such existing common legal agreements.