Dark theme
(in case of collision) "overwrite destination only if bigger"
overwrite (but if destination file different (size or date) then save destination original in collision/path/to/file)

crashing if destination doesn't have enough space (?)

Destination does not have enough space (it does if you counted the overwrites)
"replace the default copy and move system"

No, don't change the default action but ADD alternative to copy/move
Perform copy/move with multiple thread (a configurable number of threads, let's say 8 threads by default or half the number of cores in the cpu ?)

Add a performance graph  (configurable, but show speed vs time by default)

The file operation  'playlist" should be sortable. In particular I want to sort operation in order of file size (do all the smallest files first)

Check and ASK all the questions you have to ask first. If you have any questions, then place those file operations to the bottom of the list and do files you can do without asking questions. That way the transfer does not stop

On top of the copy/move options there should be a compare. This should produce a list of the files from the sources that are not already in the destination. This list could then be loaded as the operations list

Clicking "more" should not resize the ultracopier window width

If you choose skip in case of file collision, at the end of the transfer operation, you should be left with a complete list of all the skipped files

right clicking the ultracopier systray icon, should (have more options)