A buildable trench network doe'snt sound like such a bad idea--considering now in Generals if you deploy a SAM Site or any other piece of defensive equipment on the side of a hill, the terrain is manipulated to meet the width of that structure and support it. We could do trenches much the same way; some buildings already can clip into hillsides and be half buried; we could pull the trench system off because the building could dig into terrain and stay there.
Its a good idea, just would require some tweaking and support. As for Gun Emplacements, this would also work; but how about a buildable structure with this implemented. A nice PaK 36 or a '40 would work wonders against light tanks. A 12.8 PaK would work decent wonders against allied tanks. I like both of the ideas though.