The basic tenet of Islam is that there is only one god and Muhammed is the last prophet.
Hazrat Ali RA was given the title of khatam ul aula by Prophet Muhammad PBUH. Does that mean there can never be another wali on earth? There sure will , but no body will be as greater and/or superior to Hazrat Ali RA.
Likewise, prophet Muhammad PBUH is khatam ul anbiya.. His greatness can never be matched with any other prophet before Him. Islam was completed on Muslims at the time of Prophet Muhammad PBUH, but due to the changes and misinterpretations by mullahs in Islam.. Allah told prophet Muhammad that an Imam mehdi will come who will be from ur very own religion and He will be the one who will wake the true Islam , the same Islam that was once followed by people during Prophet Muhammad's PBUH time.
ahmadis also believe that there is none worthy of worship but Allah and Muhammad SAW is His messenger.