DS9 was sufficiently dark.
It wasn't a question of simply being dark, it was the strong character development that made the darkness pleasant.
Anyhow bsg wasn't that dark you guys need to get more in touch with the real world.
Anyhow I don't think star trek should get dystopian, the utopian ideals should get explored farther, but in adult terms.
Utopia in pragmatic terms isn't perfect. I think any future series should explore the true costs of living in a utopia.
Their are obvious restrictions in terms of personal freedom,
anti drugs, anti gene moding, anti capitalistic etc.
A civilization based on colonization (it's a wagon trail afterall)
should explore the environmental risks of living on the frontier. Radiation exposure, unstable planets etc. This can create a great deal of conflict, and story if done over the arc of several episodes. Prime example is the episode where picard relived a lifetime in a dieing civilization.
If it's an adult world, explore the different cultures of these different worlds. Not in a value based system, where some message is forced onto the audience, but in more of a passive secondary way, where part of the adventure is in the discovery of theses cultures.
Don't just glorify the role of a ships captain, play up the role of legal experts, paramedics, firefighters, biologist etc, this is something trek can be great at. Even an episode on train spotting can be great if you play on the right themes.
If a utopia, is free of sexual diseases, fit, and free of social constructs, explore what this means. A future where the captain believes is having relations with everyone and anyone could provide some very interest character development. If done discretely.