I think of heroes as adding depth, not as strict replacements. Examples:
Caedmon would be a good card for you. Against blue titans, you could swap him in for BT. In raids against teams with riposters (e.g. Boril) or with lots of buff casters (e.g. Alberich), Caedmon might replace LJ or another card.
Rigard’s spot could up for grabs, at least in some cases. Sabina combines Caedmon’s debuff trick with HP gains equal to Rigard, but with stronger tile damage. Merlin is a strong card both in raids and titans, if you happened to get him last month. You also want to think about what to bring against purple titans, when Rigard isn’t much use. Guardian Jackal (probably available next month) is a nice card, best paired with Joon or Chao. Even without those you’ll see huge tile hits on purple titans. Otherwise the 4* yellows are kinda meh, though Li Xiu is probably the strongest.
Any strong hitter would be a great option for your deck. You have Grimm to soften up the target, Wu and BT to +Att, but you don’t have a strong single-target hitter. 5* examples are easy: Joon, Lianna, Sartana, Marjana, Magni. 4* examples are less stellar: Chao, Caedmon, Kelile, Sonya. Event heroes are best examples of the strong 4* single hitters (Cabin Boy Peters, Falcon, Jackal, Merlin, Lancelot).