Actually, it would be a good way to END the series. Optimus sacrifices himself (He's Optimus, it just wouldn't be Transformers if he didn't sacrifice himself to save everyone) to destroy Unicron and end the war altogether; no more Decepticons, no need to introduce Rodimus Prime or any of the others.
Trilogies are nice. They have a beginning, middle and end. They need to stop at 3.