Hmm, how about if we had special stances for engineers?
(and not ACU / SCUs presumably).
Reclaim, Capture, Repair instead of the hold fire etc.
So you can set your engineer to Reclaim, and then make a patrol path or assist ground, and it will wander around reclaiming things exclusively. You could also make it smarter about it, like preferring rocks to trees if they are nearby, and stopping the reclaim when your mass is full.
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That would make for a very interesting change of gameplay - if there are very few mass points on the map it will become more of a limited-resource game like Starcraft.
An interesting bit of variety from the normal game.