I do not know wether this is really a new idea. Anyhow just my two cents....

Add a nice-looking small drop-box at the right or left upper corner of the desktop (everytime top-view). Even im the case NP++ is closed.
By simple drag and drop one can start NP++  and display the dropped file (maybe even not open but simple start Np++ in background to allow further drops). For sure it would be nice if it works with multiple files as well.

Inspired by some CD-Burning programms which use this method to create a collection for a CDR.
Just browse trough the filesystem, and drag & drop as desire to open all files.

Everyone can test this feeling by minimizing the NP++ Window as small as possible put it to the left or right corner and if you like to open a file simple drag and drop the file to this small window.

Hmmm....By this way I believe it could be very easy to implement, just a fourth icon beside the minimize, close and open icon on the upper right corner of the NP++ which minimize the window to the smallest possible box and move it to a desired corner of the desktop plus addtional allways-on-the-top property .

As i said just my two cents.....