That door terrain thing is an intriguing idea. I'll play around with it, and see how it works. My worry is that the moving objects are blocks, rather than thin walls, so placing a wall along only one edge of the block doesn't make as much sense as encircling a whole block with the door terrain marker--which raises the issue of what happens if a unit moves into that block while the door is "open" and then overrides it "shut". It's a block, so shouldn't it just lift you to the top? (The absence of elevation rules in this game have always bugged me.) It might make more sense if you imagine it as the unit being swallowed up by the block, with a trapdoor closing over it or something.
Actually, that's kind of neat, as long as it isn't in gambit. Hmm...
I actually like the door obstacle better for a freestanding energy shield concept than for the shifting block terrain (since the terrain in the scenario was controlled by the trainers rather than those in the arena). Tactically speaking, I like the idea of a big energy barrier that prevents ranged attacks on your base, but then disappears the moment an enemy moves up and tries to take cover behind it...
In that situation, override makes perfect sense, and it's a new tactical use for that ability that doesn't invoke any risk of lockout.
If I don't incorporate freestanding doors on this side, I'll make a different arena for the back just to put this idea to good use!
P.S. "All huges have stable footing" sounds like a very good idea to me.