Series 8 is one of the most interesting and unique series of Doctor Who since the relaunch but honestly, in my opinion was one of the most mixed series we've had. None of the episodes were great in the same levels as Blink, The Doctor's Wife or The Empty Child. Some were hit and miss, some even utter crap such as Kill the Moon and In the Forest of the Night and some were alright like Mummy on the Orient Express. But nothing for me really stood out.
There are many reasons for this as well as many other failings of series 8 so as such, I have a few points that could be improved or changed for the next season or any other season coming which could make the overall quality of episodes better.
Spoilers for the series 8 finale
Less focus on character arcs
Series 8 is most the character focused series we have ever had, with the relationship with Danny Pink and character growth of Clara being front right and center. The problem with that is that if you don't like the character of Clara or don't care about her relationship or story. Then it is going to be very difficult for you to fully invest with the series.
Personally, I thought Clara story arc was not interesting enough and lacked any subtly to truly invest. Now this had nothing to do with her acting but rather that many of the episodes that focused on the character arc were weak or bad as well as the writing in many of these scenes was not up to par for me to care.
For series 9, I would like to see it as less character focused and more plot focused. That's not to say character arcs should not be involved, but I don't think it should overshadow the plot like what has happened with many episodes and should be far more subtle about it. Though if you have to do a fully central character episode, for the love of god make it a good one.
Tighter focus on the theme
One of the main themes of this series was that of death and the afterlife. This is a very interesting one and one that could have been used to further develop the Doctor as well as had the possibility to be used to create interesting episodes.
The problem is that it is never focused on truly, even in the finale it was more of a plot point rather than expanding on the idea and focusing on it.
We never see any discussion of it. At the start, the Doctor does not believe in the afterlife and by the end the Doctor's viewpoint hasn't changed with no challenges to his beliefs and no debate of any significance. This is partially because the writers had already made the Doctor right which I personally think was a mistake. There is not really anyway for him to know there is no afterlife as he has not died properly himself and is now possibly incapable of doing, so it would have interesting to see him truly question his beliefs rather than him just going, "Nope."
I also think it is very lazy storytelling, it would have been better if they let the story be more open ended then just exclaim. "Anyone who believes in an afterlife is wrong and does not deserve to watch this show, now f*** off and watch flying cybermen."
The other problem is that the other themes of this series were not interesting or developed enough. As I said before, the Clara arc was not compelling, the Doctor's anti-soldier viewpoints was not really implemented well throughout the 12 episodes except in a few spots and the Doctor being a good person, though the most well-developed, felt somewhat meaningless for me as I already knew what the answer was going to be.
I think this is a less is more situation and in series 9 I feel that they really should focus and at two main themes at the max so they have more breathing space and the themes are able to be fleshed out more. I would also like to see the idea of the afterlife and death at some point to be truly get a deep examination rather than the lazy way it was implemented here.
No more cop out endings
Steven Moffat is known for his twists to his episodes. However this can lead to cop out endings as seen in series 6 with Let's Kill Hitler and The Wedding of River Song and series 8 is no different. Both Kill the Moon and In the Forest of the Night suffer greatly because of this with neither of their endings making much sense. Kill the Moon is bad because it ruins an actually interesting dilemma and Forest is a bad twist in an already weak episode that doesn't make sense in Doctor Who logic (if the trees are there to stop a massive solar flare then where were they in Evolution of the Daleks when the largest flare in 1000 years hits earth?)
In series 9 and later seasons, can we not have these cop out endings and actually go through with your plots and dilemmas. If you're going to kill Hitler, KILL HITLER! If you're going to kill the Moon, KILL THE GOD DAMN MOON!
More focus on the Doctor
Finally, this is a more minor issue and connects to my first point about character arcs. Due to the focus on Clara and her character development, the Doctor gets far less time to develop and time to understand him how he work and feels then usual. This was probably a bad thing to do in a new Doctors first season where viewers are going to like or dislike the new Doctor and leave the series for other shows.
In the next series I would like to see more focus on the Doctor and him as a character.