I would gave all my votes for only this idea, if I could.
The potential and variety of ways to use Memento is so wide, that the list of available autofill sources will serve needs of only minor part of users, as long as the size of this list is fixed by developer.
For instance, I'm a chemical scientist, and I would be happy to have websites of the American Chemical Society, Royal Society of Chemistry, and some others in list of autofill sources to save books, reviews, scientific research articles, etc., which I refer to or simply gonna read while working on my own project. These publications have lots of parameters (e.g. title, authors, journal title, year, volume, issue, pages, doi, weblink, abstract, references, referring articles, and so on), thus such database, linked with my own projects database and some others might become something like embodiment of Memento developers' initial design, as far as I understand. But it's a very hard work to create such database without appropriate autofill scource. And, of course, I realize that sources, which I need are extremely rarely used among Memento users, so it's obviously needless to add these sources in a fixed list of sources in app. But I'm sure that overwhelming majority of users has the difficulty similar to one I have. We need to create a DB, that is based on certain web source (though, which by its own is relatively rarely used), and it's pretty hard to do that without using an appropriate autofill option.
I would be glad to see such option in future Memento updates. However, I can imagine an amount of work, which is done by developers day by day, and I'm ready to wait. For today, I would be happy just to find out that such feature is going to be introduced in future))