I don't claim to be an expert on Isaac, but Psy users in his home game can use all four Djinn types, correct? On that basis alone, I feel the team could make a unique character out of Isaac that would be worthy of DLC. It seems a common theme of all DLC characters is that they need to bring something very new or unique to the table (Terry's unique fighting style and meter, Steve's mining gimmicks, Min-Min's two-arm system, Joker having Arsene, Sephiroth having wing form and Masamune, etc.) Its why most Dixie supporters don't feel good about her chances since she would likely be a Diddy semi-clone and not bring that unique flavor.
Isaac has the potential to have a fighting style that employs four elements, which is certainly unique. Closest would be Byleth's four weapons. But would a style like his be balanced, or focus primarily on Earth with others only splashed in?