My expected behaviour is that the vehicle will repath as quickly as reasonable to the new target. If that needs to slow, stop, turn, etc, then I'd expect that. Basically, I'd like it to repath on click regardless of being on a current path.
Hi! I agree, it's not optimal that you can't give different directions while driving, but having such a feature may not be much better. As you said, you'd like to repath! Pathing can take a while, especially if the new location is far away (think of selecting a new location in map mode) or hard to reach (because the vehicle can't find a path through your walls or a factory block). In order to calculate a path, the vehicle needs to be in a fixed position, so it would have to stop first, calculate a path to the new position, and then go there. It could happen that you'll sit there waiting for a while before the vehicle starts to move again. This would be really bad if you didn't want to change the destination at all, but accidentally clicked somewhere with the remote control.
I'm not sure this really would work as good as it seems. By the way, there already is a way to stop a vehicle: the GUI contains a stop button. Perhaps defining a hot-key for "Stop" would be an option? Pro: You don't have to move the mouse over to the GUI, so it may be faster. Contra: One more hot key competing with combinations from other mods. :-)