Islam Is Not The Last Revelation And Prophet Muhammad Is Not The Last Prophet
Islam was never meant to be the last Revelation. The Mystery of God will be finished after the Seventh Angel Comes (Revelation 10:7). There are evidences even in the Koran that it is not the Last Revelation. Here are some:
"O Ye People of the Book! Surely, our messenger has come to you to clarify for you, MOST OF WHAT YOU USED TO CONCEAL of the Book, AND HE PASSES OVER A GOOD DEAL..." (The Table Spread, V15)
So Prophet Muhammed revealed "MOST OF WHAT YOU USED TO CONCEAL of the Book" and "He PASSES OVER A GOOD DEAL..." So He does not Reveal the whole thing. There are things that He left out. God did not Reveal the whole thing through Him. Others have to come and complete the Revelations, reveal the rest of the Book, etc.
Surah Ornaments of Gold (Az-Zukhruf)
Yes, It [the Koran] is an essential part of the Eternal Tablet (the source of all Scriptures), which we possess. (4)
It is an essential part of the Eternal Tablet (Akashic Records) , not all of it, nor the last one and ! There will be more Revelations.
Another reason Muslims believe that Prophet Muhammad was the last Prophet is a sentence He uttered in His last journey to Mecca. He said that He has completed their religion (He is finished bringing any more Revelations to them). Muslim take this as He has brought the Last Revelation from God. What He meant, however, was that He finished what God Choose Him to do. He finished Islam and Revelations related to It.
As we saw above He did not Reveal everything. His Revelation was only a part of the Eternal Tablet. How then He could have Finished the Revelations from God? Furthermore, it is the Seventh Angel (Revelation 10:7) which will Finish God's Revelations and Reveal His Mystery. Prophet Muhammad never claimed to be the Seventh Angel. He never opened the Seven Seals, etc. So again this sentence clearly means He finished the Revelation of Islam, not the Last Revelation from God.
Conclusion: More Revelations (Prophets) will come after Prophet Muhammad.
Dogmas bring conflict. Truth brings Unity!