I'd grow me out a baby rtc and while he's in there getting big and obese I would get me a shovel and dig out a 1000 gallon + heated pond! LOL.
If I had that 450 with those dimensions now though, I would try a huge male jaguar and move all of my other stock over. Also, I would maybe try a midas pair or even dovii or umbees would be pretty sweet if it was a one species tank. Also... obligatory oscars, would definitely get a few haha.
Hmm...also.... fahaka puffer maybe with some fast tank mates? Mbu could work too if it's the 4ft I think. IDK about those puffers really other than they get enormous and require lots of filtration and tasty shelled critters. Would be pretty cool if you ever change your mind about agressive fish.
You could also do an enormous planted tank if you are up for the challenge. You could do some of the less agressive and more colorful ca/sa cichlids. Discus and angels always look amazing in huge tanks. Maybe get some of those with some big schoolers like tinfoil barbs or denison barbs too? Maybe just do schools of small things? You have a lot of options.