"It's just like this other game that I like, only with these other elements added that I also like!"
In the real world, games are rarely made that way. There are all kinds of constraints that get a designer started. Some examples:
- A publisher issues an RFP for a sequel to an earlier game, now that the original developers are out of business.
- A publisher acquires the rights to a licensed IP and asks for concepts using that IP.
- A publisher notices there are no announced titles in a certain popular genre within a certain fiscal quarter a couple years from now. You start with a genre, timeline and budget which are all written in stone, but you're free to be original otherwise.
- A publisher notices a fast-growing, underserved player demographic and asks you to make a game to specifically attract that demographic (such as the notorious "games for girls" phase that the industry seems determined to screw up about once every ten years).
- An educational/training company approaches you, asking you to make a game to teach a given set of content more effectively than traditional classroom study.
- A political organization commissions a persuasive game to push a specific agenda or raise awareness of an important issue.
- An indie developer wants to make an "art game" to express their own emotional struggle through gameplay.
I've found the best way to break students of their habit is to introduce them to a variety of real-world constraints, giving them practice in designing games to those imposed constraints. Because game ideas may come from all over, but the ones that get made into AAA games usually come from constraints imposed from above.