I would love to have a massive tank that could have every species of fish I've seen at my LFS. I know this'll never happen though, not because of the size of the tank I'd need but the rampant compatibility issues.
I would love to have a complete marine tank with a sting ray and coral reef section. Compatibility issues will arise, though - I'd want the eels and stuff to go in there! Alongside that, I'd have the perfect replica of a lake in the Amazon Basin... I wonder if you could mail-order a gravel vac for something like that...
In addition to a bunch of Hoplos and swordtails, multiple guppies of all kinds. And maybe a small group of betta females.
In a gigantic tank that is. About 1000L ...
If I had the money, I'd be going for some kind of odd build of a double-tank: 2 tanks 1 next to the other, 1 with salt water, 1 with fresh water, and seeing both fish types as if they're in same tank... Now that would be funky. LOL.
A huge tank with a sustainable population of some small fish, and a few large ones. 500 Ember Tetras and 6 Black Ghost Knifes, for instance. Am I a cruel person?
Sometimes I wish fish could just... float through air. That would fix the space problem... but it would raise another problem: looking for your fish under the furniture or making sure you keep your windows closed so the fish won't float out of the house.