Your first pass at researching the market for your game should involve three of my favorite tools.

1. SensorTower: for researching keywords and game types similar to your own
2. App Annie: for understanding when they were launched and how they have managed the rankings.
3. Think Gaming - To understand what kind of revenue you would typically see in the paid/free/grossing rankings.

Sensor Tower Keyword Searches: This will help you identify games that will compete with you for search volume. Try to identify at least 10 search terms people will use to find your game.

App Annie historical rankings tells the story of how competing apps have done in their respective categories from a download volume, and grossing standpoint. You can use this information to determine where your app needs to float in the rankings.

Phase 10, a popular card game on iOS ~ 500 paid downloads/day and ranks top ~100 paid US-Games. Breaking ranking milestones like Top 100 and Top 50 means even more visibility and increased downloads from browsing.

Think gaming provides you with download estimates on top games based on their rankings. These three tools combined offer a lot of insight into game download volumes, revenue assessments, and more.