@alastairp mentioned that “after fonts, it might be a good idea to review other external links too” and there was some brief comments about known 3rd party vendor JS scripts being used in other projects.
For LB/MB, I would really like to see this one solved seeing as LB will be of increasing importance in MB’s future: https://tickets.metabrainz.org/browse/MBS-11586 (could very well be too complex for GSoC)
For BB, I recently found out that Goodreads retired its API. Maybe BB should find out what Goodreads had that it does not and replicate it? Seems like a lot of people were really mad. I’m not sure if this is within GSoC scope.