You can turn a layout into a fragment.
And if you edit the fragment you can add parts to its regions.
However if you use the layout fragment on a page, you cannot add parts to the layout fragment.
We sorta need a new concept “layout template” in which the layout can be stored and reused, but the parts are added to the “page layout” not the “layout template”.
So, basically, you want to use a fragment (based on a layout), then “customize it”, basically turning it into a regular layout again?
I want one person (tech savvy, say web master/editor) to be able to create and save a bunch of “template layouts” (without programming).
I want another non technical person (content contributor/journalist) to be able to choose among those layouts.
This is basically fragments - but additionally you would like to customize them after adding them (diverting from the original fragment)?
Caveat: I gotta mention this “idea” sorta got spawned from a non technical person and reached this forum without much thought process between 
Anyways, here’s a little more detail:
I have a grid layout in which I can build any dynamic grid (similar to bootstrap).
Selecting the correct values for breakpoint, columns and offset requires a bit of understanding of how the grid work.
In addition we don’t necessarily want to give “journalists” the ability to build any grid.
But rather we want the “editor” (without having to involve developers) to give the “journalists” a few predefined grids to pick from.
This might belong more in the category of templates rather than fragments…
It might even be possible already, I just haven’t tried.
Or I’m “confused” because I’m thinking outside the box of CMS
Can one make a page template with layout and perhaps even some parts already added?
If those layouts and parts where “readonly” on the page and all a “journalist” was allowed to do was add parts to the regions. Then I guess I got what I’m thinking about. I guess the complexity would be, what if an “editor” decided to change the template later, what would happen to all pages using the template.
I understand the part fragments “contain” the parts config, and if you edit a part fragment (changing its config) that change “reflects” everywhere the part fragment is used.
Now for layout fragments, one could store only the layout configuration as in empty regions and not the “content/parts”. Then one could use the layout fragment multiple places, but add different parts on each usage.
I guess I’m mixing the “cards” abit, but wouldn’t it be cool to be able to do such things in Content Studio rather than having to “hardcode” each layout.
Many questions in one here, let me try to answer:
So you want editors to build “layouts” - why? The reason why layouts are shipped in code is to control design and appearance - this is not something you can be sure an editor will handle much better than the average contributor.
You can make page templates that are intended for being configured every time, i.e. 5 different templates for landing pages. It can even contain placeholders for components - i.e. here you should have a part, some text, an image etc etc.
Currently when the user starts customizing a page (based on such a template) he is basically building a 100% customized page, and the setup from the template is completely overridden. The reason for this approach, is that it is very complex to visualize and practically handle the inheritance you desire (just changing parts of a page). We might introduce capabilities in the future that enable things like this though.
Essentially, a part is just as much a page component as a layout - for instance if you add another part above a layout you are changing the position (and thus path) of the layout too - so we would need unique identifiers for each component added to the page in addition to path. More advanced permission handling might prevent a user from editing/removing layouts from a page I guess, but as mentioned earlier - practically handling such inheritance is hard for both the editors, contributors and the system itself.
We have added a feature to enable “exploding” a fragment on the page. This way you can use the fragment as a template to get started.
