Slowmo by dragging clip handles
Resizing a clip changes its size. Pitivi does that by simply shorten the part of the clip or the start position.
What if there was a modifier key, let's say Ctrl (or alt) which would change that behaviour when pressed while dragging the start or the end of the clip? What could happen then:
While holding down the modifier key and changing the length of the clip, Pitivi could keep the length, but stretch or compress the same content into the new length.
For video that would simply mean a change of its tempo.
For audio that could either be a simple tempo change or - by choice - timeshifting. This can be handled by scaletempo.
There is barely a way to accomplish such tasks easier and quicker.