Toadfish (freshwater lionfish) are great oddballs as well. Although mine was always kept in a marine tank so I have no experience keeping them in full freshwater. Super fun fish to feed though!
leaf fish, black ghost knives, ropefish, bichirs, pictus cats, wrestling halfbeak, pike characin, plecos, a piranha tank (not really odd but they are eye catching), pickerel, all the species listed above too are great options