Have interesting, compelling, well thought out and engaging characters, rather than having to fill the helm with a series regular, squeezing in a character of a different colour, or making it all about the boobs.
It'd be interesting to see a new series scale back the number of main characters. Saying that however, DS9 had by for the biggest cast of main and recurring and was also the most consistent with development.
Plus the big explosions helped
I do agree that DS9 probably was the best in terms of character development for the most part at least.
As far as the size issue is concerned I'm not really fussed about whether or not it's a small cast or a larger one like DS9 as long as it's a new Trek series that's good
If I had to stick my neck out however I don't think they'll do a small cast of 4-5 I think their more likely to do a similar size to the likes of Enterprise or Voyager.