I posted a thread about the Siblings. There are lots of opportunities for the show to have cameos from different characters, but to have new cast members full time would suck. The writers have opened up enough opportunities to have visiting family members to add more faces to the show if they want, but can make them single appearances so as to NOT have another situation like Pria being on the show for so long. Someone referenced FRIENDS early in this thread. The early seasons were great because you didn't need any back story to enjoy the show. But once they started having relationships, you had to be familiar with the cast and their stories to understand what was going on. I hate that. The storyline of the 4 geeks with their quirks and the hot neighbor is what got the show it's fan base. Cameos from characters like Stuart and Zach and Kripke are all great to show there are more people in the world of Big Bang and the inject of people like Levar Burton and Will Wheaton and Brent Spiner and Katie Sacoff and George Takei are all great cameo appearances for the geekdom fan base. Throw in a cameo by Carrie Fisher or someone from Star Wars and see the fan base erupt with joy!