We should display the fields necessary in the editors, and only fields that are needed for the source type. If "Author", "Abbreviation" or "Pub Info" are not needed, we shouldn't display them.  Likewise for "Date" and "Volume/Page" for citations.
I have a problem with this. That's because in my experience the idea that source type can define needed fields is not supported by reality. Proof is that results of the same type, say some civil birth/marriage or death record found on our national Wie Was Wie site, do not only depend on the type B/M/D, but also on the data provider, that is the software used by the local (mostly provincial) archive that supplies the data to the national site. Moreover, the fields available on the archives own site, may be different from the aggregated ones available on mentioned national site.

Adapting EE types to the above, and this is just for The Netherlands, is a hell of a job, I think, and if that leads to say a 1000 types for the whole world, which is very likely, when you want to cover sites all over the world, is just crazy.

I have RootsMagic here, which I acquired to connect to the FS tree, but when I try to add a source in that, the choice of templates simply scares me away.
I agree, but when things become complicated, we need or a bigger editor windows to show all, or some sort of guided data entry.

My current thinking is:

1/ Instead of the tab 'General' for source and citation, we show the tab 'Overview', which would have only few fields editable that make sense, and then show concise the important things.
I like the idea of an overview, but because of the above, I have no idea how to select fields that make sense.
3/I would like to enable some copy paste function though on the Definition tab. So, I would like to offer some mechanism to quickly copy paste or select existing parts of title/pub info (for users fixing imported gedcom or old gramps sources), and to import a bibtex and select fields from that. Perhaps a bottom part with buttons, or drag and drop to a top part with the actual fields? Need to try some GUI ideas for how to do this.
Having a lot of sources that need fixing, this is something that I really like.
As to Title, Pub Info and Author, perhaps best to make those just Attributes and deprecate them. Same for Page/Volume in citation. Date is special, as allowing the use of the date editor is not something we want to loose...
I would really like the author to become more important, and on the long term implemented as the agent in the GedcomX draft. Author/Agent then refers to say a local government or church that issued a document, or a relative (person in Gramps) that provided info by email.

Title comes next in my view, i.e. I like to organize sources by author, so that I can distinguish church books which have very generic titles by themselves by their author. Depending on citation style, the combo may be displayed in any order, specified by template or user.

Page/Volume are also things I like to keep, and Record number too. Currently I have no idea where to put that.

Pub Info is a field I never understood. Was that meant for ISBN or so? No idea? Sometimes I put a URL in there, but I don't think it was meant for that.

Which brings me to that URL. I definitely want that on the standard screen.

On FamilySearch, every item in my Source Box has 4 fields: Title, URL, Citation, Notes. That's all. Adding Author, Volume/Page, Record Number, Date, and Source Text, would result in 9 fields that cover about everything I can think of right now. Guess that's my generic template then.

And in this context, Citation is the scientific term, that string that can be F, L, or S, not what we have in Gramps, or know from Ancestry, PAF, and so forth.