I have two dream tanks.
The first:
A free-standing 360 degree acrylic tank with dual center overflows, centered in a room surrounded by smaller tanks (display, QT, hospital and frag). 12' long, 36" wide, 21" tall. Mixed reef tank with multiple zones spaced throughout the tank - high light, high flow zones for an acropora paradise, shielded lagoons with tall rock walls and lower lighting for LPS and euphyllia gardens. Rockwork throughout, filled with multitudes of caves large and small, populated by rock-dwelling smaller fish while larger reef fish circle throughout the tank.
Small, niche fish like pink-streaked wrasses, basslets, chrysiptera damsels, shrimp gobies, tilefish, etc., serving as "peak-a-boo" fishes, little things that you see when you go in close and really watch the rockwork. Wrasses and anthias constantly circling through the tank as dither fishes, with enough volume to support harems of flasher and fairy wrasses and schools of anthias.
On the larger side, schools of tangs for algae control. A quoyi parrotfish, because they're amazing. A pair of crosshatch triggers. Small harems of Genicanthus angels, and a couple of the large showier angels, like an Emperor, Koran, Majestic or Regal. Pyramid butterflies, copperband butterflies, etc.
If it's a true dream tank - probably lit by metal halides over the acro zones, LEDs for the LPS zones, with supplemental T5s throughout. Tunze ORCAs for flow, along with strategically placed gyres and point-source powerheads for flow in specific areas. Also possible some submersible powerheads hidden in the rockwork to keep things stirred up.
The out-of-tank equipment would be housed in a separate dedicated room, with easy standing access to all equipment (no crouching!). Probably a large cryptic zone in the sump, along with a display macro refugium and a large algae turf scrubber. A calcium reactor for major element control, and multiple dosing pumps with short hose runs for each individual trace element. A recirculating skimmer with automated draining, and an automatic water change system set up to change out 2% of the water every day.
The second: Most of the planning and such would be identical, the equipment would be identical. But rather than a freestanding tank, it would be 6' tall pane of acrylic, much deeper as well. Basically this, but as a reef tank: