See there's "money is no object" and then there's "No seriously, you're Bill Gates"
And that said why do I have to maintain it? Money is no object, I'm hiring someone to clean that sucker
That said one of my favorite tanks I've seen (that isn't Steinhart big) is this outdoor "pond" tank that I've seen at the Maui Ocean center as well as the Waikiki Aquarium. Basically is about 3.5-4 feet tall acrylic panel from the ground up (it's been a while so I may be misremembering size) it had some angles in it and the rock structure was absolutely huge with plenty of spots to move around etc, it really makes the idea of "piece of the reef" seem a bit more realistic compared to what we have in our tanks which is a sliver of the reefs at most... and then a really weird reef at that
, but I always thought that was a super cool idea. Steinhart actually has something similar to that when you come down from the rain forest dome except it has freshwater rays in it instead so obviously different scape. Even thought about doing something like that at one point, except inside instead of outside, but quickly dismissed the idea as the evaporation from something that would be big enough to be worth building (not the same if it was just a 100g tank on the ground) would be pretty substantial combined with the heating required (and freshwater top off required) would make that quite expensive in this area, maybe I lived elsewhere where electricity is reasonable... sure, but until then. No.
The crazy insane, no seriously I have no issues with money, tank would be something like I saw on a Youtube channel Ohio Fish rescue where he converted an indoor pool into a huge tank. Granted it was freshwater, and there probably would be quite a few issues involved with getting it salt water ready "Yeah Marco rocks? What if I needed 50 tons of rock... no seriously, 50 tons" but I think having a diving reef would be kind of fun (This is less deep version of the Steinhart reef tank).
That said the "Free tank available" dream tank, probably would want to stay under 500 gallons, maybe closer to 300 gallons, but I'd want it long (8 feet at least) width 30" inches at least unless I can find some of that super awesome rockscape stuff that goes on the back wall (not a wall of rocks, but the back wall is just a solid rock formation), then I could do with 24" width possibly. Having a look down shallow tank could work nice... although with LEDs it's often a pain to look through the top I find.