Time seems right to debut him at Night of Champions. Just from watching this build, I really don't see Styles losing this first match, and I don't see him defeating Cena clean, nor should he if he's going to be a top heel. It makes sense to bring Balor in and ally him with Styles, Gallows, and Anderson if they want to debut him in the most impactful, effective way possible.
Than again, I have a strange feeling that they're going to want to get some mileage out of him as a face on the main roster before any sort of turn. I just have a hard time believing they'd bring Balor in as a heel and as a second man to AJ Styles after all the effort they've put into him and "The Demon".
So basically what I'm saying is, I don't know... There have been many times when I've been almost sure of what was going to happen, and what I thought doesn't even come close to taking place. It's very possible we don't see him until after Takeover Brooklyn. As a side note, it would definitely be cool to see a heel Balor vs Nakamura at Takeover. Much more intriguing than a face/face match in my opinion.