My colleagues said not bad ideas so I decided to post them here.
1. Make optionable use of temp file on other drive i.e. if you need
to defrag stuffed drive with low amount of free space available,
but you have some hard drive with a lot of space on it so you may
use temp file with custom size. It'll be great in situations when you
come to person with stuffed drive with your own and also it'll find
it's usage for regular defragmentation.
2. Make hard and soft stop. It means Hard Stop to stop defrag
aborting last action and Soft Stop to wait till last started operation
is done. It'll be good to wait till it's done if you wait for a long time
(for example movement of dvd image)and don't want to repeat
this waiting by aborting this operation.