WOWZA what an improvement!
You'll have to decide between big fish or little fish. Because ever so often, big fish like to eat little fish.
If you want some ideas, you can look up rope fish, bichir, peacock eel, fire eel, African knife fish. Less of a handful than the lung fish or the rays.
If this tank is at least 2 feet wide (pretty sure it would be), you're one of the few lucky contenders for caring for a black ghost knife properly. Really common fish but most of them end up in awfully small tanks.
Also could have a lot of goldfish in there.