Write a module. Like an expansion pack. There are tools within most games nowadays to create your own levels. Start out by creating your own levels, using someone else's engine (the toolset in neverwinter nights).
You can make multiple levels, link them together and create your own dialogue. This, to some degree, will let you create your own game. There are limitations doing this, however, if you do a good job, people can download your level and play, thus giving you feedback.
