There is currently no queueing file copier in Gtk world (AFAIK), UltraCopier working only for QT applications or catchcopy v0002 protocol compliant ones.

The idea is to make Thunar a file-copy manager able to queue copies instead of doing them in parallel.

It would help to pick up (for instance) 15 movies from an external disk, one by one (speaking which to pick up between to starting copies) to copy on local disk, without having 15 parallel copies and no way to exactly know where it stopped when it crashed (may be because the USB plug was pulled). I lived this scene many times, it involves at least 2 peoples, and leave a bad image of GNU/Linux…
The other scenario for this activity is : carefully grow a selection which will fail (because of a mouse handling mistake) at the 15th file, loosing the selection just when you wanted to start the copy.

Dragging a new file to copy would add it to the file-copy queue, and everything goes well.

Here are the main features of UltraCopier : https://en.wikipedia.org/wiki/Ultracopier
-    Pause and resume the copy
-    Copy list can be edited during the copy 
-    Apply speed limitation
-    Resume the copy where it would have stopped if error occurred
-    Collision management (overwrite, skip, cancel or rename) (this we already have \o/)