One that interests you or others?
Here's a hint, do a market survey and make sure there is a need for the solution you want to produce. Find out what already exists out there, and how it stacks up to the solution you want to provide. Will your application be sufficiently differentiated enough to carve out it's own niche?
If not, back to the drawing board to come up with something that has a conceptual fit and an existing demand. Too often I have witnessed a lot of programming hours wasted on coming up with an application that had already been on the market for a few years. Worse still, some of the existing applications were far superior in terms of ease of use and capabilities than their latest challenger.
Note if you think that games development is the way to go; I hate to rain on your parade, but it's a very risky market to go after. You are literally rolling the dice in hopes of coming up with something new. Besides, according to recent reports, there's a lot less money in the games market and that business applications is where the real money is at.
In short, follow the money. Find an industry that has a need for a new application and be the first to solve that business need or problem with an application that is easy to use, and you are well on way to success. But whatever you decide to do, make sure you do enough research up front to verify that the market you think, hope, or dream is there, is actually out there.