Wa alaikum Salam.

The Our Beloevd Prophet SallAllahu Alaihi Wa Sallam is the Final and last Prophet

Muhammad is not the father of any of your men, but (he is) the Messenger of Allah, and the Seal of the Prophets: and Allah has full knowledge of all things. 
(The Holy Quran, Al-Ahzab 33:40)

The Hadith states, “My position in relation to the prophets who came before me can be explained by the following example: A man erected a building and adorned this edifice with great beauty, but he left an empty niche, in the corner where just one brick was missing. People looked around the building and marveled at its beauty, but wondered why a brick was missing from that niche? I am like unto that one missing brick and I am the last in the line of the Prophets.” (Bukhari, Muslim,Tirmidhi, Musnad Ahmad, Tirmizi, Babu Khatimin-Nabiyyin, Musnad Abu Dawud Tayalisi)

When Hadrat Isa Alaihis Salam will come, he will not come as a new prophet nor with a new Shari’ah. He will descend from the skies and will follow the Shari’ah of Our Prophet SallAllaho Alaihi Wa Sallam.

As regards to Hadrat Imam Mahdi RadiAllahu Anhu, he will be from the descendant of the Holy Prophet and a ‘Hasani’ Sayyed. He will be an Imam and a leader.

Allah Knows the Best

Moulana Mohammed Hussein

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First of all, Imam Mahdi will not be a prophet but a muslim and a follower of Our Beloved Prophet. 
Second of all, The Prophet (sallallahu alaihi wasallam) is the final Prophet to be sent by Allah. There shall be no prophet after him. Prophet Isa (Jesus) was taken up to the heavens by Allah and is still alive in the heavens. When he returns towards the end of time, he shall return as a Muslim and a believer in Our Prophet (sallallahu alaihi wasallam). His return shall not involve him starting a new religion.

And Allah knows best!
Zakir Hussein