I would love to see a browsing experience on KOReader similar to that of Nickel. One that takes full advantage of file metadata.

Desired features
Ideally, this interface would allow the user to:

• Browse all on-device files in a single list
• Browse by author
• Browse by book series
• Browse by Calibre tags
• Browse user-defined collections
• Filter by file format
• Filter by reading status

Why the file browser is insufficient
At present, the Cover Mosaic view in the file manager is the closest we can get to such an experience, but it has its shortcomings:

• It can sort books by read status, file type, etc, but it can't filter out on that same criteria
• KOReader can search for Calibre defined metadata like author, but it's not possible to browse
• The only way to get a complete, browsable list of all files is if they're all located in the same directory. The only way to get a browsable list of all authors is if files are already sorted into the corresponding author's folder. These are mutually exclusive.
• The "History" view allows the user to browse a list of recently opened files regardless of their location on the device. The problem with this is it the user must have already opened the file. The window also takes up the whole screen and must be closed in order to access the top bar.

Limitations of this feature

• This feature would require indexing all the files of the main directory. The current file browser is quick because it is limited to the current directory.
• Some Ereaders may lack the processing power to complete such indexing within a reasonable time, especially when dealing with large libraries with potentially thousands of books.
• Not everyone manages their metadata. This feature is useless if files don't have the necessary metadata.
• The existing file browser meets the needs of many users. Don't fix what isn't broken.

In light of these considerations, I think such a UI would be best implemented as an optional plug-in and NOT a replacement for the current file browser. The current file browser is satisfactory for many users, but not all users. Implementing this as an optional feature would allow all users to use KOReader that works best for their preferences and their hardware.