If you have a (very) good idea on how to improve Notepad++, you can request a feature using IdeaTorrent. You can also use the Feature Request tracker (http://sourceforge.net/tracker2/?group_id=95717&atid=612385), even though this facility may be phased out at some point. Using IdeaTorrent allows for competing solutions for a given issue, and voting for these so-called solutions. If your feature hasn't been asked for before, you can do so here, or vote for an already proposed solution.
There is no guarantee it will be added however; what is a great feature for one person might be a big nuisance for another, or it simply is too difficult or time consuming to add it in. They are hints as to what are the most wished-for additions or modifications to the software, and they will be considered on the basis of urgency, popularity, available developer time and technical feasibility.
Also, some feature requests would involve modifying the Scintilla component, which would prove unpractical. In such a case, the feature request would be better filed with the Scintilla project. When this project upgrades the Scintilla component, and when Notepad++ can take advantage of the new versions, all the new features become available - they may have been asked for by people who never heard of Notepad++ -, and the large tester pool for Scintilla will have probably weeded out any possible bugs therein.