Another plugin idea: Searching for songs via meta data.
For example MP3 files may have so called ID3 tags which contain information like Album Title, Track Number etc.
I have no idea whether it takes much performance to search for a given text in many MP3s but perhaps it's possible. Creating an index is possible too (many music players do that, then called Library or something similar).

I tried to browse into some of my folders but the program fails when it comes to umlauts!
See the attached screenshot.

Another example:
Entering "aufge" returns the folder "_Aufgenommenes": "I:\Öffentlich\Musik\_Aufgenommenes\" and pressing enter opens the folder.
BUT if press the Right arrow key after selecting the entry FARR goes [as desired] into file browsing mode but NOW [see screeny 2] pressing Enter doesn't do anything.