Requested feature overview description.
When querying favorites of a user fav:tilcreator the results are always displayed with the default order order:-id.
There is currently no way of changing the order to how it is displayed in /favorites.
The proposed order:fav would order the posts by the added to favorites date of the queried user (fav:X).

Why would it be useful?
First of all it would allow to have the order changed to the added date at all when querying other users favorites.
[~Bug] But also with the new next/prev feature on /post/X the order of the posts on /favorites and the order that is used for the feature is different.

What part(s) of the site page(s) are affected?
https://e621.net/help/api#posts_list
https://e621.net/posts.json
https://e621.net/posts
https://e621.net/posts/X

It is unlikely that I can implement this short term. The underlying search system doesn't have good support for joining data from different areas together, so it doesn't know what order favorites were added in, only a list of which users have marked a post as favorite.

The favorites page was updated to work around this by pulling the favorites directly from the database instead of through the search system. I will be looking into if I can add search back into the favorites.

You can view other users favorites in order by visiting their profile page and clicking the number of favorites they have. I understand this isn't as convenient, but it's a work around.