This is a great idea. Hope you will use time and really rewrite the program. sftp is almost built into Linux these days (OpenSSH), so why not try and make the program support it?
And when you're on it, why not make another version for Linux? For what I know there's only one good GUI program for linux, IglooFTP Pro, which also is for Windows.
You prolly want to beat them. As their Single-User price is higher then yours, FlashFXP is better looking, easier to use and so on.
Edit: Typos.