I know it's a couple years early to be talking a about 6th but it is an inevitablity we all must endure so I'm puting my opinion in on what I'd like to see changed.
Namely what should be counted as a defensive weapon on a vehicle.
in previous edition it was strength 5 or less which was fine by my opinion
in the current edition it's now strength 4 or less which is also fine however some weapons become defensive that shouldn't e.g. frag/plasma missiles and vice versa some weapons are no longer defensive that should be e.g. heavy flamers.
I think the following rule would be a better solution:
Rapid Fire and Assault (Edit: and Pintle Mounted) weapons are defensive, Heavy weapons are primary.
quite simple if a standard infantry model could fire the weapon on the move unaided then so can a vehicle.