The completeness of Islam obviously means that it cannot be completed further. Believing otherwise would imply one or more of the following dangerous conclusions:
That Allah was not truthful in declaring this completeness.
That Allah has forgotten or missed some details needed to complete the Deen.
That Muhammad (peace be upon him) has neglected or forgotten to convey some matters needed to complete the Deen.
This explains why Islam warns strongly against introducing bid`ahs into the Deen. Imam Malik said:
"Whoever innovates in Islam what he believes to be a good bid`ah would be claiming that Muhammad (peace be upon him) has betrayed the trust (of delivering the full Message)..."