Why does there even need to be a heir to Batman, does he even want one? Who says the cowl is come mantle to be passed down, what if Bruce's intention is that the Batman dies with him and no one else takes up the the identity? I think Dick is content as Nightwing and the other Robins could/should find their own unique identity as well. Honestly I don't think Bruce wants anyone to be Batman after him and he would prefer they do their own thing. knowing him he probably has some protocol in place to seal up or destroy the Batcave after he's gone.