Hello itimpi,
I'd have an urgent feature request - don't know if this is possible, though, however I think lot of people using calibre2opds for a cloud ebook storage will run into this problem:
There are too many folders generated over the time. I have about 2,500 ebooks resulting into around 1,000 authors. Updating is not possible anymore because I get constant errors from the cloud service that I've exceeded the storage limit. In fact I have only exceeded the maximum numbers of folders possible.
Even when you use a cloud service that has no such limit, updating lasts hours because the folder structure is read over and over again. So my suggestion would be to insert a layer between the main folder and the authors, such as literals or something like that, e.g.
eBooks
----A
------Alice Bob
------ ...
----B
----C
.... and so on.
This would cut down the problem dramatically, because each literal folder would contain maybe 20 to 100 subfolders, that are read within a blink of an eye.
Another cool feature would be a link to the author page where ever the name of the author appears.
And finally

- it would be cool to have an option in between yes and no for 'Do not generate cross-reference links'. I want cross reference links, however they don't need to reflect the current state. It does not matter if there are 789 Thrillers or 792. This would avoid that every single book page is updated again when you add new books.
thanks!
Cyana