You are right about /næbæʔ/
نبأ meaning novel information that may have been past. My understanding of it comes from the hadeeth of the prophet, peace be upon him, in which he describes the Quran. Among the descriptions he said:
فيه خبر ماقبلكم ونبأ مابعدكم وفصل مابينكم
Meaning, "In it is news of what was before you, prediction of what will be after you and the decider of what is between you."
I'm now thinking I should use the word I used to translate the name of the chapter, i.e., Announcement!
Eureka! At least I think so.
/næbæʔ/
نبأ should be translated into
prophecy. It dawned on me that the Arabic word is derived from the verb origin /næbæʔæ/
نبأ from which the word for
prophet is also derived. The context here is clearly discussing a future event (hence your suggestion of
prediction).
I even suggest that the translation of the name of the chapter be "
The Prophecy." I was a bit concerned that
prophecy might be taken in the sense of "Being a prophet," but I think this is less of an issue since
prophethood is the matching word for that.
BTW, you convinced me that "asking one another" is a more appropriate, safer translation than "wondering" (also, "asking one another" sounds to me to be more suitable here than "asking each other," since it gives more of the sense of wondering). I was also convinced that starting the first verse with "About" is better than delaying it because it matches the Arabic structure without sounding odd in English, and also matches the second verse. My conclusion:
{78:1} About what are they asking one another?{78:2} About the great prophecyI'll think about 78:3 a bit more.