This is also a way for WB/DC to cash in on the Crazy Sci-Fi cash from the people who like Star Wars or Marvel's Guardians of the Galaxy... Also It's Batman!
My choice for Terry McGinnis is Leo Howard. You may have seen him playing Young Jason Momoa on Conan: the failed Reboot, or a young Ray Park on G.I. Joe: We now use Ironman suits because screw the source material, Baroness is Cobra Commander's sister!
The kid has a Background in Martial Arts, which can help with the action scenes as the new Batman...
Of course, the difficult part would be casting Old Bruce Wayne, but let's face it: There is only ONE CORRECT ANSWER:
Michael Keaton IS the PERFECT Old Bruce Wayne. Out of all the Batmen (post-Adam West, Pre-Bale) he's the only one who knew that Bruce Wayne is one messed up dude.He'd be PERFECT to play Old Bruce.
I'd love to have Chris O'Donnell on the sequel (inspired by Return of the Joker.) and Alicia Silverstone as Commissioner Wilson (she wasn't Babs Gordon on that horrible movie)
It works to give some closure to the Pre DCCU Bat movies... kinda like Superman returns, but less sucky. I could have suggested for WB to do a Static movie, buuut DC is afraid of trying stuff outside The Big Two, so that's also a reason for having Batman Beyond. It allows DC to experiment WITHOUT breaking out of the Big Two.
But what do I know?