What’s the Mahdi going to do if he arrives? 

According to Hadith, the Rasul of Allah (saw) says; A revitalizer will come in the beginning of every century…

What will the Mahdi proclaim if he does come?

He’s not going to tell us anything other then the verses in the Quran and the Hadith of the Rasul of Allah (saw).

The Mahdi will remind you of the Quran.