I dont think JR should be aiming for the "File Manager" space, but instead (if file management is an area of potential) aim for a "File Management System" (best description I can think of right now).
There are lots of "file managers" out there and, as pointed out, you'll always be competing against those who have a direct interest in that space. There is not too many "File Management Systems" out there though. Sure, there are DMS's out there, but they're not really aimed at the home user, and generally speaking, really do only cater for documents.
Ideally, I'd love to see an application that can apply some organisational intelligence across a wide range of file types (docs, music, video, books, wine reviews, software installers, recipes, etc etc). Optimally, the application would be able to provide access to said files both from and to distributed locations and across multiple platforms.
What I've always wanted is an app that can seamlessly integrate files stored across all our PCs into a single coherent navigation system that can also be accessed remotely. To clear it up a bit, imagine an MC-style app that can take a library stored on my machine, join it with a library on my wifes machine, as well as the master server library, and present said libraries in a single virtual library. These libraries that I speak of should also not be limited to music, imgaes and video.. oh no! I should be able to find our tax returns where ever I've lost them, the wife should be able to access her recipes, stored on her PC, from a lappy in the kitchen, while listening to music from the server, with some recent photos I took (that are stored on my PC) playing as a desktop background wallpaper - all from the one management system. And a nice web front end would be good too (local server, and remote PHP based too)
Plugins would be a must for integrated file display (ie pdf) as well as for reading meta info from particular file types.
I'm sure this is all pie-in-the-sky hopes for the next few years but really, its just a selection of what I think a File Management System should be able to do (further features include, but are not limited to.. replication, backups, archival, ....). If you want more suggestions along these lines just let me know, I'll happily oblige you