A couple ideas with regard to terrain:
for use in scratchbuilt/converted terrain as armoured door for a bastion or a storage facility.
Old Cities of Death components, probably some of the new kill team terrain components also for conversions.
Sector Mechanicus terrain, e.g. Galvanic Magnavent has some components which are really nice for a huge promethium refinery. Or other industrial buildings.
Some of the Pegasus terrain, thinking technobridge in particular (if still available).
I also have various polystyrene bits from old electro material, lots of interesting shapes there.