Since you said tank size and cost are not a problem, id try to get a couple pair of taimen ( endangered trout that can get over 6' found in Mongolia) probably not possible regardless of how much money you have.
A huge tank with several pairs of golden dorado would be cool.
Maybe an Amazon river based tank with a few pairs of each of the largest catfish; rtc tsn jau and Piraiba
RedTail Catfish, Dovi, Fresh Water Ray, Jaguar, Peacock Bass, Large Mouth Bass, Fly River Turtle, Shovel Nose Catfish, nothing to special since honestly I think certain extremely large species I would feel as though nothing is big enough for them at least acrylic or glass tank wise if I am keeping fish I want them in a tank not a pond just my 2 cents