I'll be honest, season 8 was disappointing to me. The episodes were fine, but nothing stood out, and the real test of how much I liked the episodes is how many times I watch them. I only watched season 8 once. There are several episodes in every other season of Doctor Who that I've watched more than three times. Some are approaching ten times. Yet, I can't even remember half of what happened in all the season eight episodes and haven't felt like going back and watching again. Season 9 looks like it has the potential to have some re-watching episodes in my future once again.
At the San Diego Comic Con, Jenna Coleman said that the overall tone of the show this season is darker with really thought provoking episodes, which you can see from the trailer. She also talked about what is coming up for Clara after losing Danny.
"Yeah, I think since she's lost Danny, she's kind of cut ties a little bit to Earth, her perspective has changed. And all she wants to do is travel and have fun and meet some aliens and enjoy everything time and space has to offer."
She also said that Clara now has an "adrenaline-seeking high" which leads to "some danger along the way."
"But it's definitely the glory days of Clara and The Doctor."
Another great reveal, especially for Game of Thrones fans, it looks like Maisie Williams will be guest starring this season. After that little snip-it in the trailer, I'm very curious as to who her character is.
Source for Jenna Coleman interview: franchiseherald.com