This might sound cracked, but I wonder - and this is a very vague idea - would it be possible to put an ‘Arc’ section in the Drafts folder of projects like Screenplay, the various radio and play scripts and so on? In the same way there’s a section where you can noodle around with characters, thinking who they are and what they do, maybe an Arc section could have… umm… maybe Inciting Incident, First Block, Crisis, Solution, Denouement…?
Why not create it in the Research part of the binder?
I think overall Scrivener should remain as agnostic to method as possible, in the templates. We already go far enough in suggesting a folder/file arrangement of “Chapter” and “Scene” or what have you. It is very easy to mistake examples for rules, when you’re learning something new. Consider how many screenshots you may have seen around here of draft folders that consist of 30 folders all with one single “Scene” file inside it, like wrapping each individual pinto bean before putting it in the jar. I mean, it works, but that’s about all you can say for it. 
But stuff like this makes for an excellent example of how to (a) create a project from a built-in template, (b) modify it to have whatever structural or other elements you prefer and © save it as a new template that you now start from—optionally (d) upload your template if you think others might find it useful.
It’s your project. It’s totally possible to create whatever you want, and put it wherever you want.
And, if you find this approach helpful, you can save the project as a template for future use.
Katherine
It might also be worth searching around this forum a bit where plot development has been discussed off and on and some have posted project setups they have used along the lines your talking about. It’s been awhile since I was in one of those discussions here, so I will have to leave the hunting to you.
Since things like this are, usually, nothing more that a series of folders, they are easy to set up once and and drag and drop into different projects as wanted. I think this is a better approach than baking it into your project templates.
I maintain a Scrivener project called ‘Writing Tools’ and if some structure like that seems useful enough I will set it up in Writing Tools so I can simply drag and drop it’s governing folder into a live Scriv project for use there. That also facilitates developing a set of such things which may be useful for different purposes.
gr