I love Mp3tag for it amazing capabilities
but there's one thing I would love to have:
I would like to use Web Sources in Actions and I'm thinking of this way:
In an Action I want to define which a Web Source (which I already added into the client)
- the parameters I would like to pass for the search in the Web Source (e.g. %Artist% %Title%)
- the parameters which information I would like to get (%Year%; %CoverArt%; %Genre%)
- the parameters to which tag the retrieved information should be stored (%Year%; %CoverArt%; %Genre%)
For the case if multiple results are returned for one search, then there should be (hardcoded?) rules which apply - e.g. take the record with the oldest %Year%, etc.
This would help me a lot, since I would not have to use the Web Sources for each song manually 
What do you think about it?