Just throwing an idea onto the table, cos it looks about the right time period (and apologising in advance if it turns out my Batman lore is not up to scratch), but I think a really good final villain for this game would have been a newly resurrected Solomon Grundy. Maybe you feel his influence throughout the game, battling grunts in his employ, fight him in his human form, then the finale of the game has you square off against him as his regenerative powers set in. Solomon Grundy vs Steampunk Batman... what do you rekon, Escapists?