Learn Spanish (Spain): El director exigió que el vestido fuera innovador. - The director demanded that the dress be innovative.

The main verb exigió is in the preterite (a past tense). According to the sequence of tenses, if a past tense verb triggers the subjunctive in a subordinate clause, you must use the imperfect subjunctive. Because the demand happened in the past, the second verb is locked into the past subjunctive.

Is it also correct to say que el vestido fuese innovador?

Yes, absolutely. The imperfect subjunctive has two sets of valid endings: -ra (fuera) and -se (fuese). They mean exactly the same thing. The -ra endings are more common in everyday spoken Spanish in Spain, but you will frequently encounter the -se forms in literature, journalism, and formal speech.

Could we use an infinitive here, like exigió ser innovador?

No, because there are two different subjects. The director (el director) is the one demanding, but the dress (el vestido) is the thing that must be innovative. When the subject changes after a verb of influence like exigir, you must use que followed by a conjugated subjunctive verb.

Why is fuera (from ser) used instead of estuviera (from estar) with the adjective innovador?