I'd prefer if we would have hooks for community scripts, for example a checkLegal - hook providing the entered license, source and author and the community script can decide whether and for what reason to reject the upload.

This way we don't have to hack every single Commons-Feature into UpWiz. Does this make sense to everybody?

Although I cannot think of a wiki where having files from Flickr without source URLs I also think that UploadWizard should be more customisable by each wiki using it. However, I am afraid that this is not going to happen any time soon as it would mean a lot of work which would possibly become useless after the major update this Q3.

(In reply to Stöger Florian D. M. from comment #2)

after the major update this Q3

Where can I read more about this? What will be updated?

There is no official announcement, [[mw:UploadWizard]] only mentions an update in 2014:
https://www.mediawiki.org/w/index.php?title=UploadWizard&diff=880464

However, there is a section in a RFC on MediaWiki.org where Gilles Dubuc gives insight on the "UploadWizard experience […] that's been collectively agreed on for the next version":
https://www.mediawiki.org/wiki/Requests_for_comment/UploadWizard:_scale_to_sister_projects#UploadWizard_isn.27t_going_to_be_as_much_wizard-like_as_it_currently_is

Work on UploadWizard is scheduled to begin in May, although that depends on how the MediaViewer rollouts go. See http://lists.wikimedia.org/pipermail/multimedia/2014-March/000118.html for some details.