Oh yes, a swimming pool sized tank, at LEAST! that you can swim in and also see in from the outside.
Would be cool to have a ton of little fish (corydoras, tetras, etc.) and watch their more natural schooling/shoaling behaviors due to the sheer massive area of the tank. The behaviors/activity levels seen in the wild are much more different and more entertaining/interesting to witness compared to how fish act in our relatively small tanks. And yes, swimming with the fishes would be the best experience, even swim in an aquatic jungle of underwater plants haha)
But of course there are so many different stocking ideas I have in mind, it's hard to decide which I would want most. (from small peaceful fish in a massive tank to large predator only tank or even a nice paludarium with fish, turtles, frogs, crabs, and other amphibians coexisting together peacefully, as well as all the crazy looking plants).
But more realistically (more likely to obtain a long time from now), maybe a 180 gallon or larger. Or maybe a large koi pond with a arowana in it (don't want to give up yard space though :P)