I think that most users use just the basic functions and are pretty much happy, because they work. You would have to come up with something revolutionary to get them excited and that's hard to do, you can't sit and invent something fantastic, just like that. But there are tons of things that can be improved or optimized. No "wow effect" from that, but personally, that's exactly what I want, making things better, without following some stupid fads and making "changes for changes". Too bad it's not the "in" thing nowadays.
If I could suggest something for TC, then off top of my head (random order):
1) Truly asynchronous panels. E.g. when I need to wait a minute to open remote directory with several thousands of files over slow VPN, I want to be able to do something else in the other panel or start a program from button bar in the meantime. Instead of just staring at "locked" TC and waiting. I realize that this is probably big one, implementation wise, but it would be really cool.
2) Allow plugins to use "<number> drive" feature, just like internal FTP. You know, when you connect to FTP server and you have it accesible as drive "0". And second one as drive "1". Unlike with e.g. SFTP where you connect and must take care to not reuse the panel for anything else, otherwise you have to take another trip through network neighborhood, when you need it again.
This one annoys me for years, more and more in fact, with increased SFTP use. In just two weeks, I'll have
10 years anniversary with this one. :) I don't care much about centralized "bookmarks" any more (but I wouldn't say no to that), just the "<number> drive".
3) Support for stuff like links. I know it's not for most users, but advanced file manager should be able to create a stupid symlink without external tools.
4) Various optimizations. E.g. when comparing files by content, file A can be read from disk in 4 seconds, file B takes the same. But comparing them takes 30 seconds (non-SSD drive, depends heavily on file's position on disk). I'm sure there's possible to do something about that.
5) User interface. Now don't get me wrong, I myself am die hard fan of classic Win9x interface and already decided to be a founding member of "Windows 7 (with Classic theme) forever" club. ;) But the fact is, TC was born in days of 640x480 and it did not change much since then.
Is crowded ~550x450 settings dialog really an advantage? Or various other dialogs with fixed size, e.g. the packer one, where file/path field is only ~350px? It was fine in 8.3 file names days, but now it's not that great any more. Or that stuffing of checkboxes in the same dialog, why? Copy dialog got a width update recently, but it's still fixed. What if someone likes really long names?
These are of course details and TC will work great even if they never get touched. But it would be nice improvement if they did.