"in the opening recitative of the coronation of poppea, which word is repeated in several different ways, sometimes sounding minor, sometimes sounding major? "

I believe the answer is: Tornerai

In its original language, the word tornerai has the meaning of something that being revert back to its previous condition.
The word itself does not oftenly used in day to day italian language, but in the opera, the word reflected the true feeling of the characters.