I think in the end Chris will either end up dead or back in the Phantom Zone. De-aging him back to 6 after his relationship with Thara would be extremely creepy and having him be an old man would be as well. Besides, DC can talk all they want about keeping Dick as Batman when Bruce comes back, given who's now in charge over there (Diane Nelson and Warners) by the time the next Batman movie debuts, Dick will be back as Nightwing and Chris will be gone.