I know there is a mod for this by the name of AAI Automated Vehicles, but that system is too complex for the vast majority of people. So I'd like to suggest adding in trucks for carrying things, and having their automation work off of a waypoint system like you see in Red Alert 2.
In addition, if you added tank automation in with this you would have a system for auto biter clearing of a set area, removing an aspect of excessive combat in late game when you are essentially all but invulnerable. You could even consider throwing in a biter like AI system so that if set tanks will target biter nests if they're set up to do so.
I thought of this because right now, except in the very early game all that tanks are really good for are clearing trees. They can't even launch nukes, and their armor is weak. But if you have a full out battalion of 20 or so tanks going at it and gunning stuff down they suddenly have a use for automated biter removal.
All in all this would add to the game greatly because it would add another aspect of automation to a game dedicated to it
