I just have one tiny request: The option to invert the direction of the flip.
Currently, if the flip direction is vertical, then downward pen movement translates to backward frame flipping, upward is forward frame flipping. To me, the other way around is a lot more intuitive (imagine navigating a vertical dopesheet). My hand just wants to move down in order to flip forward in time. It's seems to a pretty strong habit to overcome since after a few years with TVPaint I still need to stop and remind myself the direction is the opposite of what I keep trying to do.
So a couple more options in the "Direction" pulldown menu would be great:
"Vertical Inverse"
"Horizontal Inverse" (this one would probably be very counter intuitive to most but... it would make the list complete
)