A feature that could be added is the ability to "shred" or "wipe" the files after they are deleted. Basically, shredding a file just means writing data patterns over the drive sectors that the deleted files used to occupy. For instance, writing all zeros, then all ones, then random data, then specific patterned data, etc. When you've completed those steps, it's nearly impossible for anyone to recover any of the original data. Of course, you should also let the user choose the level of shredding...the average user might just want the data erased with an all zeros 1-pass, since that would be the fastest option, while certain corporate users might insist on a 3- or 7-pass DOD algorithm. At any rate, it is not difficult to enable file shredding, especially at the level that a normal user would find useful.
I believe this would add a lot of value to your program, and would make it stand out from the competition.