synchronization is not an issue (in my opinion) at all!
there is good freeware out there, eg.:
http://freefilesync.sourceforge.net/
why reinvent the wheel? consider that with every new feature, plenty of new bugs and old friends (regressions) will be (re)implemented!
until synchronization works properly and reliable in order to use it in a production environment months will pass ... why not focus on fixing already implemented half-baken, buggy features?
my solution: better support & implementation of 3rd party tools in general:
- Sync (freefilesync)
- FTP (filezilla)
- Hex-Editing (HxD)
...
with one things you're wright, quick filter is a / THE mayor feature, other FM lack!
the feature I miss most is drag & drop support for favorite folder and favorite items!