It occurs to me that you can use such a setup to keep another monster fish for life under amazing conditions, in crystal clear water, and with the ability to observe the full extent of its natural behaviors. I, personally, would not find it enjoyable to have built a tank that will suit 99% of fish species, only to worry about long-term housing because I bought one of the very few animals that are certain to outgrow it.
If you picked some large catfish or cyprinids, for example, or kept e.g. peacock bass, you would have a stunning display tank, no long-term concerns about how to house the animals, and the appreciation (and jealousy) of the forum-goers here. As I understand it, tanks and fish nowadays are judged not by their size or rarity, but the care they receive and (especially on online spaces) the frequent notes and observations about the goings-on of the tank.
Whatever you end up choosing, please do keep us updated.