The next Star Trek
series will be just as dark as its venue requires. On Netflix or TNT, not so dark. On Cartoon Network - an animated series that is dark by kid's standards, but not by grownups. On Showtime - dark enough to be perceived as consistent with their lineup (Dexter, Homeland
), so pretty dark. On Starz (which just announced a space war series), very dark, violent and sexy.
Quality is a different issue. You can have a good, dark show or a good, light show. Obviously, nobody intentionally creates a crappy show, but the pressures to make something high quality will be strongest on Showtime, less strong on TNT, and as for Starz, I'm not at all sure they care.
Another big unknown factor is the showrunner who is the driving force behind the series (and there will have to be someone driving this project, to overcome CBS's inertia about it). Bryan Fuller would create a distinctly different series than Seth McFarlane.
But everyone can stop worrying about TNG - the TV ecosystem that supported that kind of space opera series is long since defunct. There will never be a Star Trek
series like that again.