Something that I think would go well with landships: The ground could be an obstacle to weapons. The ground (not trees or floating islands) could block bullets, cannon fire, flamethrowers, etc. This would give landships a slight advantage over stationary fortresses, because they could retreat behind the landscape if needed. But they would still be vulnerable against airships.
I think that this would be helpful, in addition to the reasons described above, if you place your building In a hole, it's subterranean part should be only really damagable By destroying the land around it or bashing through the top of the building to get to it's "basement"
That could be pretty interesting, yeah. And it would give a bit of an excuse to include arching, indirect fire weapons in addition to the direct fire ones already in game. These would be mounted on the top of the vehicle, and would do more damage (partly because of heavier shells, but also because the top of things are rarely as well armored as the front), but would have a minimum range (within which they cannot hit the target) and lower firing rates (due to heavier shells).
On that idea, Indirect fire weapons would also have more damage because the shots that they fire would be able to accelerate on the way down, adding more energy to the impact than would occur from a direct fire weapon with a similar structure.