Serra's Sanctum and magical mirrors I am in the initial planning stages for a LEGO build of Serra's Sanctum from Magic, The Gathering. Part of the description states:

Serra's Sanctum was a notable landmark of the plane, suspended somewhere in its infinite sky. The Sanctum was the dwelling place of the famed planeswalker herself. It was massive, taller than most structures on other planes. Powerful artifacts run through white mana and having white-related functions, such as the Cocoon, could be found here. At the top of the Sanctum was the aviary, where Serra could see the entire plane through many powerful lenses that kept an eye on her world. Only planeswalkers could comprehend all these images simultaneously. Leading down from the aviary through the rest of the Sanctum was a spiral staircase that ended at the base of the building. It was here that the main plaza was presumably located. The plaza contained a water fountain that reached five stories high, carved with skillfully rendered angelic effigies. On the fountain were also the words art, discourse, freedom, peace. These are the ideals that Serra herself sought after and wished to center her world around.

It's the 'powerful lenses' part that I could use a little help with.  My current idea is to use Plate, Round 4 x 4 with 2 x 2 Hole  with some transparent-clear plates backing it to make a mirror/lens look.  Do you folks have any better ideas?
I like your idea. I know very little about the MtG universe, so I'm not sure what might be most helpful here. Some ideas that come to mind:
The 2x2 boat stud elements are available in trans-clear and might provide the sort of look that you are hoping for.

There is a mirror element (20193) available. This could potentially be used in a lens apparatus of some sort.

The 2x2, 3x3, 4x4, and 6x6 dish elements are all available in trans-clear, and they could be useful for designing a lens system.

There's also a variant of the 6x6 dish with a handle on the side instead of the studs in the center that might work better for your application.

As zovits pointed out, the 6x6 also comes in a half-sphere variant:

There are also many printed transparent dish elements that might provide the sort of look that you are looking for, such as something like this: