Hebrew is the only case in history where a concentrated effort actually succeeded in making a language with no remaining native speakers be spoken natively by millions once more. There are even (although very few) Hebrew monoglots in Israel.
What other now-extinct language could similarly succeed in regaining a stable population of native speakers?
I exclude languages like Czech that were "revived" only by becoming a language of culture again, even though it never stopped having millions of rural speakers, and languages like Manx or Cornish, which while having a very small revived population of native speakers, is almost certainly incapable of challenging the near-total dominance of English and return to being Mann and Cornwall's primary language.
That is, I want this revived language to be the community's primary language once again, as Hebrew is in most of Israel.