Quel est l'équivalent du subjonctif futur (« were to » en anglais) en français ?
Unless I'm wrong, the only common use of “were to” in modern English is “if [someone] were to [do something / have done something]…” (or similar with subject–verb inversions). I'll assume this is the use you are interested in.
The most generic translation is maybe:
À supposer que [cette personne] [fasse cela / ait fait cela]…
But in specific contexts I can imagine quite a few more translations.
Si je devais / avais dû…
S'il se trouvait que…
In old times when subjunctive was a full-fledged mood in English and before conditional appeared in Romance languages, they might have been more closely related, but what is called subjunctive in modern English (or what remains of it) and what is called subjunctive in modern French are different enough, that you shouldn't expect a one-fits-all equivalence in terms of tenses and moods.