be the tip of the iceberg
To be only a small, often unrepresentative portion of something much larger or more complex that cannot yet be seen or understood. If Congress doesn't vote to extend the debt limit, a government shutdown will be the tip of the iceberg in terms of what the country will have to deal with. With mobile apps, you have to account for replay value, monetization, ad revenue, and how much the players will spread the game to their friends. Making a fun game is really just the tip of the iceberg!