The reasons you gave are actually why
I think it IS the Batcave lol
In the scenes from BB where Wayne Manor is being destroyed in the elevator shaft system you can see the metal grating is very much like the kind in the debated "Team Cave or Sewer" photo. Also as I've said in previous posts I've never seen a sewer that looks like that before, not saying their aren't any, I'm just saying I've never seen one like that. Especially with such good lighting and multi-tiered, with clean looking waterfalls...which we also saw in the BB Batcave. Since the new cave will be decked out with all sorts of new gadgets and must fit The Bat (theoretically) etc, etc. I think it's a strong possibility that its the Cave. But I could be wrong