I think that it would be great to enable custom size of the arrowheads which is not dependent on the curve they are added to. For example, when I change the thickness of the lines in a certain display mode, arrowheads assigned to curves via the ! _Arrowhead command are scaled up or down accordingly. However, that makes them really ugly in some situations, for example when I use custom display modes where the curves are 5 pixels thick, hence the arrowheads get really huge on my screen. Being able to set a custom size for the arrowheads will be welcome! 
I suggest to integrate this in the individual display mode settings, so that switching to another display mode will not destroy the default look of the arrowheads there. That way, the user will be able to set different arrowhead size for each display mode and not worry that they will look ugly or hard to read due to the difference in curve thickness across the display modes.
For example, this image contains curves with 5 mm thickness, along with their duplicated with 2 mm thickness with added arrowheads, in order to replicate the look of the Gumball’s Move arrows. Before doing that trick the arrowheads on the 5 mm thick curves were super ugly:
Another request is to make the arrowheads displayed before the ground plane and the shadows, because currently they get partially covered by those effects. To make the arrowheads visible in this sample image I had to change the selection colour from yellow to white, select the lines, hide the Gumball and then make a screenshot.