Currently, the Discovery Search team is focusing the new cross-wiki search results to be gathered only on the sister wiki projects.
For example, when searching on fr.wikipedia - cross-wiki results could be shown from fr.wikivoyage, fr.wikiquote, fr.wikiversity, fr.wikisource, fr.wikinews, fr.wikispecies, fr.wiktionary and fr.wikibooks - if results are found in those sister projects.
Wikidata can show really pertinent results.
Also, have you noticed Extension:ArticlePlaceholder?
Hi @Trizek (WMF) - I chatted with the team and we feel that Wikidata is very excellent and a lovely pivot that we can maybe use for searching across languages- it might be a good candidate as a companion to our language detection efforts.
But for the cross-wiki search functionality, we really don't have any control on what data is actually searched for - because everything is merged into one big field. So, we don't think Wikidata would be a good fit for this particular functionality.
Note that some sister projects are very small and using them will give very poor quality results. With few and poor quality pages on a sister project those will end up on a lot of searches on some other project. That could trigger a lot of discussions, and prompt inclusion of a user option to turn the results off.
A solution could be to set a threshold on some kind of quality on the included hits, thereby limiting the number of pages where a certain hit can show up.
This could also give more incentive to clean up the low quality pages on other projects, so it could be "a wanted effect" even if it will create a lot of discussions.