It would be very helpful to have the option to automatically mark watched videos for deletion after a given period of time just like we can already do with recordings…
I love Channels and am so thankful for all of your hard work!
Do you mean imported media, like imported Videos, Movies and TV Show episodes?
Yes. I host my downloaded YouTube videos on Channels and it would be nice if watched videos could be marked for deletion instead of having to remember to manually delete them…
This is a very interesting scenario, quite unique, I would say. 
I would be very surprised if this ever became a feature in Channels.
In the meantime, I'm pretty sure a script could take care of this. That's my opinion.
I was considering some sort of a script as you suggest but came up blank as I can’t think of a way to mark watched videos in such a way that I could incorporate it into a script…
If you create video groups properly, you can use the Channels API to retrieve the list and attributes of all videos in a specific group.
I think this would be a good starting point.
Even better, you can directly get the list of watched videos in the group!
Basically, add ?watched=true at the end of the URL.
Once you have a list of file IDs that you want to delete, you can send a command to the server (at your own risk!) to put those files in the trash.
This is something we have wanted to do for a while, but have yet to do it for technical reasons.
Basically, the pruning rules on passes should be available on TV Shows and video groups. Then, even imports and videos would get the pruning features.
So it’s in our heads, we just have to solve some other things first.
As for a script, you could easily just take a set of video groups, get their IDs, then read through all the files that have that group ID and delete the ones that are marked as watched.
I won’t go into the details on how to do every step of this, but this is the strategy that would work. Everything is there to make a script like this.
I have an idea of how to create this script, however is it possible to create a webhook for the trash command for the channels server? In my case it would be the trash command for each video group which I have created and wish to run the script?