Drush has a very powerful alias architecture allowing the definition of custom aliases which contain details for database and files directory, including ssh connections to remote servers. Wouldn't it be cool if we could work into D2D the ability to pass in an alias as the source connection argument.

Something like this:

drush mi MyD6Site @myd6site

So that in your migration config/settings.php you needn't provide source connection details?

This could be useful for many reasons, but in the future we could also look at

drush mfs @myd6site

So that you didn't have to define the migrations before you could take a look at the source database fields.