Brainstorming encourages free-flowing thoughts and collaboration among team members to bring new and innovative concepts to the table.
Look around you
This might not look like much but when you sit back and look around you can actually see a lot of things going around you. Games like subway surfer, angry birds are just daily activities turned into games.
Get Game ideas from the internet
There are many forums where people discuss game ideas. There is a subreddit for this and almost daily a new idea gets discussed there.
Ask existing customer, friends and family
Consider taking advice from your game-playing friends and family. Try to reach out to your customers and ask them what they would like to play next.
Read Books and Watch Movies
Reading books and watching movies can give you a lot of ideas.