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
Look at the character list on Mad Med
. It's not the number of characters, it's how you use them.