Your situation is far from unique. Most of the developer jobs are actually maintenance jobs, even though usually they include at least some new features too. If there are no feature requests for your product at all, maybe it is on the road to being abandoned... you should clarify this with your manager. It is important to know why there are no new features: is management unwilling to invest any more in the development of this product, or are the users completely out of new feature ideas? In the latter case, it is also possible that there are very few active users and/or they have grown so accustomed to the way the program currently works, that they can't even imagine any change in it. However, if management is not totally closing out new feature developments, and you have contact with the users, you might be able to catalyse them to request new features. Take any opportunities to discuss bug reports and possible solutions with them, ask their opinion about how best to handle specific error cases, and how this program could be made better in general. Eventually they may mention ideas out of which new features can sprout...
At any rate, discuss your concerns with your manager. If you are lucky, you will be understood and you can work out an expectable solution together with him/her. E.g. it may be possible for you to work part time on another, actively developed product.
If not, you basically have two choices: continue your current job and work on pet projects in your free time to keep up and improve your development skills, or start looking for a new job.