Definitely the lungfishes due to size
Northern Pike--same issue
Sturgeon---size and availability/legality
not a fish but would love to keep the hellbender (giant aquatic slamanders), but impossible due to rarity and size (3 ft. or so)
Atlantic tarpon--size
Atlantic (SW) wolffish--size, availability
Last but not least, the giant squid (Architeuthis sp.) for obvious reasons!
I would love to get a bumble bee grouper from wes! That is one of the coolest freshwater fish to me, but I don't want to have to worry about trying to find a home for it when it gets too big and lost the $1000+ for the fish.
Chili Red Asian-cost and legality
FO Moba Fronts-lack patience to grow up or money to buy
Red tail cat-size, could built a pond, but wouldn't work in IA