The fact that Bruce left/replaced the Batsignal has nothing to do with who John Blake will become.
All it does it show Gordon that Bruce is alive out there -- somewhere and it's symbolic in Gotham's faith being restored, properly and is symbolic of Gordon being able to have a clean slate and fresh start after protecting Gotham under a lie for the last 8 years.
John Blake can become Robin, Red-Robin, Nightwing or Batman... That much is really up to interpretation. Hell, he can just become a regular, noble citizen.
Who John Blake will become is really the only thing that is left up to interpretation in the end of the film... The rest, like, how Bruce survived, how Blake found the Batcave, where the new signal came from, where are the pearls and how did Alfred find Bruce are all really dumb and pointless questions as they were ALL answers IN the film itself.
I swear a lot of people DID not pay attention to or actively watch the film. This isn't addressed to you -- this is just a not to the forum/thread on the whole. There are a ton of dumb questions coming in and MOST of them were answered in the film -- MOST of them explicitly stated, in fact.
People saying that Alfred halucenated seeing Bruce? Bruce is really dead? That Alfred 'faked crying at the funeral' so people would think Bruce was truly dead as a nod to The Dark Knight Returns? Are you kidding me with this nonsense? Some of these theories are the dumbest things I have ever heard. The ending was the ending. It was laid out explicitly.
-Alfred and all of Gotham believes Batman/Bruce Wayne died.
-Wayne Manor is given to the orphanage/Gotham City.
-Fox is told by his engineers that Bruce Wayne fixed the auto pilot six months ago -- letting Fox know that Bruce probably survived the explosion. He was able to eject early, and have the auto pilot take the Bat out to see.
-Gordon finds the new batsignal, left by Wayne, as a symbol of the fresh start that Gordon has with Gotham City.
-John Blake receives the coordinates, a GPS, and the materials to find the BatCave in a bag left to him in Bruce's will.
-Alfred continues to visit the same cafe he told Bruce about in the beginning of the film in the hopes that he may still see him. Bruce retires in that specific area because he knows that Alfred will eventually be there and he wants to fulfill the wish of his oldest friend. So Bruce goes to this cafe often so that he will one-day run into Alfred so that Alfred knows he is alive and that he has moved on and has a normal life. Bruce even has the pearls with the tracking device that Alfred could have followed to find him.
That's the ending. It's simple -- but damn near perfect. There is VERY little interpretation required. There was no "it's all a dream." That is about as silly as you can get.
But the fate of WHO "Robin" Blake will become is the only thing that can only be truly guessed at...
-R