Is your suggestion for improvement related to a problem? Please describe.
Importing data from DOI or ISBN sources usually includes a citation key which differs from the format used in the library. There is also a similar case for files, where there isn't anything one can get online.
However those fields are displayed in the import merge dialog where one needs to check each field if it should be included or not.
Custom import filters via a java class are way out of my abilities and I don't know if they could be used in online imports.

Describe the solution you'd like
The process of getting information online could be eased if there was the ability to customize the import of certain fields.