I'll make a new release pretty soon to have the new stuff tested by more people
If you have some last minute patch or small bug to be fixed, now is a good time
Exiting news! My absolute pet bug is the remaining issue in Qt5: the content of a maximized window "unmaximises" on video start (http://avidemux.org/smif/index.php/topic,16955.15.html) when loading or closing a video leads to a very broken display. Ideally Avidemux would detect that it runs in a maximized window and completely abstain from any resizing in this case. A fix for this bug would ensure quality parity with Qt4.
Then I would do ask to commit the objectively wrong but still helpful workaround in http://avidemux.org/smif/index.php/topic,16955.msg75956.html#msg75956 (http://avidemux.org/smif/index.php/topic,16955.msg75956.html#msg75956) ("bigger-window.patch") as it makes Avidemux window wide enough for the video to fit in horizontally without the need to resize the window manually.
Finally, I dream about Save the visibility state of widgets (http://avidemux.org/smif/index.php/topic,17006.0.html), but understand that it would require a lot of work and knowledge I can't contribute myself.
Two more bugs which might be easy to fix:
append resets audio codec (http://avidemux.org/smif/index.php/topic,16973.0.html)
x265 Configuration - empty / blank "Tuning" not available (http://avidemux.org/smif/index.php/topic,16558.0.html)