...
Heavy Sappers:
All in for the idea of removing Vickers at heavy sappers upgrade. Just make the upgrade cheaper and give them the ability to build demo charges (like commandos) or throw satchel charges (AI/AT?). Would give more game options and fit the idea of heavy engineers. Maybe add a flamer upgrade, so you can use them on building heavy maps in late game as a heavy fortification assault unit (do they still have the armor upgrade?).
The idea of heavy supper with flamer scary since the units has great target size and armor and the only thing keeping them back so far is the speed penalty.
Actually the upgrade should be mutually exclusive with the doctrinal flamers