I like your idea, but let say if you could get any tank you want with every equipment you want, and you can make the water parameter like what you wanted to BUT you can only put 1 species of fish there, what will you pick?
Firstly, I won't be including marine/salt water fish because there are too many beautiful and interesting species in marine.
Secondly, it won't be very interesting if you only have one type of fish.
I always keep community tank with many different species which make them more interesting and different species carry different uniqueness with them.
And you need bottom, middle and top level fish to make your tank more lively.
But if I really want only a single species, then I will choose a group of schooling or shoaling fish.
I think I might choose a group of Discus with Red, Yellow, Blue/Turquoise, and with different patterns such as Tiger stripes, Snakeskin, Rafflesia, Eagle, Leopard, etc.
But they require high maintenance.
My second choice will be a group of
Discrossus Filamentosa.
My third choice will probably be a group of Panda Loaches as they are the most active fish as they swim and climb over every part of the tank.
Here are some videos: