Too derivative of Superman. They need someone who can stand on his own. Sure, Cyborg is more associated with the teams he was on, but he can be built as a solo star. He already has a strong theme (technology) that's unique to him in the JL. STAR Labs is also an element that can be explored and provide a good supporting cast to him. I really don't think good writers would have much trouble coming up with exciting takes on Cyborg.
This is why I really hope they don't replace his solo for a TT movie. I'd be all for a Teen Titans movie, but not as a replacement for DC's only solo starring a black character. They say they're committed to diversity but that sure as hell doesn't send a good message.
Ideally, DC should make a Cyborg movie AND a Teen Titans movie. I wouldn't want him as a permanent member of the team because he's already in JL, but they could use his solo to set up the TT movie. Maybe establish his friendship with Dick Grayson as the inspiration for the team's formation idk.
Teen Titans could also be a great way to introduce more diverse heroes to the movie universe. Along with the usual suspects like Robin, Starfire, Raven and Beast Boy, I'd add Mal Duncan, Bumblebee and Bunker. Other legacy characters could be used in a Young Justice movie or series later on, but I think TT needs to stand on its own mostly.