As Chinmi said in an earlier post, password-on-bootup would be nice. Okay, so you can do it with an xbe, but this won't stop you booting games (or booting an Evox installer CD and removing the password xbe). In the BIOS is more secure, if your Xbox gets nicked you can at least relax in the knowledge that the robbing bastards have gained no more than a big doorstop (unless someone reflashes or fits a new mod, but any reputable installer ought to refuse to do so unless the customer can prove ownership).
This one should be relatively easy to implement, as the Xbox already knows how to read it: the ability to format the F: drive using GDF instead of FATX, thus overcoming the 42-character limitation without having to resort to the Avalaunch method (not that I'm knocking it) or having to squeeze in NTFS/FAT32 support.