If Megatronus Prime had survived his attempted beheading by Sentinel Prime, the timeline would be significantly altered. He would likely have been able to fight back more effectively, potentially defeating Sentinel and the Quintessons. This could lead to the Matrix of Leadership being given to Megatronus, making him the new leader of the Transformers and bringing peace to Cybertron. The events of the Megatron D-16 transformation and his subsequent actions as a Decepticon leader would likely be drastically different, with Megatron potentially being more subdued or at least having a different path forward.