I honestly don't think a flux change is the proper solution, if you increase it too much the card will be completely dead, its that it can do so much for that cost, dropping base from it or requiring domain to play it I believe will be the most effective curb but not kill.

Requiring one domain is no difference at all and requiring two would really marginalize its usefulness.

Actualy I may have to shift my view, i think Mahal MAY be fair, look at the other elemental buildings, its the only one they have with any range, they dont have a wide area big drop, thundercloud reach while awsome, is not a base, and is very expensive, all in all there all one vision buildings, none with particularly high stats. The reason its dominating is its the best of the choices in all situations. Not all decks can make use of the mogi nest or the thunderhead reach. There options are cut a little short compaired to others, I recall there being some rare location but i can't say i've ever seen in play.

It's not neccisary that mahal is endlessly superior but the fact the other elemental buildings are niche more then they can be used universal, will you play thunderheads or nest hands down, nope, but mahal is versatile for many situtations, and that capacity makes it worth the risk. Even when not playing rush out decks mahal tends to be the better choice because the others don't fit the bill quiet often.

After playing Altren's infamous elemental deck, I think one of the best ways to nerf this is to change its vision to 1. If I remember correctly, this vision 2 nonsense allowed Altren to capture the center flux point in the SECOND round. I think it went vapor mahal, vapor mahal, mogi nest for the cap. Luckily I was able to start laying waste to the rapid advancement with a thistle falcon, a treent, and two bear strengths. By the time I was able to completely turn the tide Alt had 12 or 14 glory... a tough tide to turn...

Craziness.

So I play-tested an ele deck with three vapor mahal, 3 mogi nest, and 2 magma chamber. I was able to capture the center flux well as Altren did in the game I mentioned earlier... but I realized this REALLY overextends you. If your mahals start getting attacked, you need to cast a magma chamber or mogi nest to hold your original flux well... and you get so caught up trying to maintain your infrastructure that it is difficult to find a good balance between creatures and buildings. In this situation, if you're playing against anyone with an adequate number early game creatures, you'll be in trouble.

Thus, I am no longer convinced that this card requires any change.

As I said at start
It's hard to play with Mahals so they are good as is.

I still think that Mahal's should be one sight. You could make the Orb of Radiance two sight which would mean it might actually see play in a deck.