Some people try to copy existing games because its the safest financial bet. Design is often standing on the shoulders of giants, but its not plagiarism. Look at all the game clones in the app store. Do you want to be a leader or a follower?
Can come from anywhere. A movie, a book, an experience. Study people and identify what is meaningful to people. If an experience is meaningful to someone, most likely there are many other people who will also feel that way. These experiences can be translated into games. If you’re lucky your players will experience it as you intend.
Perhaps you have a specific platform that you want to design for. Know why you are choosing that platform. What are the implications of this platform? How will the users interact with your platform? If you are using a phone how will users hold it and what will their fingers be covering up during gameplay? Do your research. Mobile games are typically consumed in bite-size chunks of time. Design your game with that in mind!
You should always be researching games. This means you have to play games and watch other people play games. A true designer sees design in everything. You’ll begin to notice things that the naive designer won’t pay any attention to. Be wary as this might remove some of the enjoyment from playing games if you’re always analyzing them. Seek out opportunities to observe gamers in their natural habitats. Ask lots of questions. Listen.
You can’t prevent having game ideas or ideas for mechanics. Don’t let yourself get fixated on any one path at least in the beginning. Remember that ideas are cheap. Every part of a design process is important, but insights are what are going to drive your project until the end. Notice how many ideas the heart (your emotional attachment) covers up.
If you aren’t solving the world’s problems, you’ll need some conflict (a problem to solve). There are a million questions, directions, and answers. Its your job as the designer to trim it down to a point that is manageable. Break it down to a core and then build it back up again. This means making decisions that will stick throughout the development process. Know why you make certain decisions and stick to your guns.