I think it would be beneficial to have an option to move specific files, similar to "move to end of disk", to after all other files, without a size limitation.
This would allow users to specify files to put after everything else that are known to often change and therefore cause defraggler to move other unchanged files when defragmenting. It would speed up future defragmentations by rarely ever having to move unchanged files.
I would also like to see this have the ability to control the sort order of the files being placed after all other files as some files may grow wildly in size (such as the tempdb.mdf and templog.ldf files in SQL server) and should probably go after the other files so that the drive head does not have to move far to access the different parts of these files.