The CSV import would have to be extended. It currently does not allow
you to import sources and citations, just simple notes. Importing
sources and citations is essential to me.
That can be done, but I don’t think that it’s really needed. That’s because GrampsXML already supports sources and citations, and it’s not too difficult to create.
Another thing is that, when importing data from an on-line source, I really think that event and person details should be include too. That can be done as text, but IMO it will be better to include those as structured data, in GrampsXML.
What I mean to say is that when you download a birth certificate, you can create an XML thing like
        <event date=... place= ... type=birth>
            <person role=child name=...>
            <person role=father name=...>
            <person role=mother name=...>
and modify Gramps in such a way that it accepts a structure like this, where event and persons are sort of embedded in the source.
The above is not GrampsXML. It’s just my way to describe a concept here.
Can you tell which site you were originally thinking about?