What does it mean to be more adult?
Will the show pay taxes and raise children?
Or is this that weird thing where TV shows act more immature with extra violence, cynicism and nudity but people confuse that for being adult?
Star Trek has always been for adults. It's actually been for everybody. That's a good thing, not bad.
How about real stakes and actions that have real consequences, rather then gimmicky plots and planets of the week that never get visited again?
Frankly more adult should deeper story telling, a plot that builds rather then one that just ends rather an hour and better characterization so we don't get useless wastes of space like Harry Kim.
The cause of Star Trek imploding in the early 2000s; it relied too much on the same old story cliches from the past, Enterprise only became interesting once they ditched the alien/planet/anomaly of the week and replaced it with a ongoing story with real stakes.
Something like Batman v Superman was dark and was garbage, Logan was dark and was amazing, because it told a good story, so again how light or dark something is, is not as important as how well executed a work is.
I think this show should be darker then TNG or Voyager, but should still have the light hearted moments that made Star Trek fun. That's why I have no problem with Harry Mudd appearing, having a goofy villain from the past could be change from some of the darker episodes we may see.
But there should be a main villain, rather then villains of the week and that villain should be one of two things: a complex nuanced villain that the audience likes or a truly scary and menacing villain that provides some high stakes, no more random jerks who have no redeeming qualities, but are not menacing in the least, who exist just so humans can act smug towards them and make some belabored point about how much humanity has progressed.
Evolve or die, this show can't be just another TNG retread, but it does not have to be GOT either.