Its got quite a life left in it. You're right, there is so much still left to do:
- Pirates Summit
- Luffy vs Ao Kiji
- Zoro vs Mihawk
- Strawhats vs Blackbeard Crew
- Finding out about the "D."
- Fighting the other Admirals
- Fighting the Shichibukai
- Shanks / Whitebeard meeting
- Getting to the last Island on Grand-line
- Becomming the Pirate King
- Making a map of the world
- Finding All-Blue
- Finding out what One Piece is.
- Learning about the "True History"
- Being chased by more Marines / Bounty Hunters
- Having Luffy's bounty hit the billion mark (ya never know
And there's plenty more I'm obviously forgetting.
One Piece will live on for years! If a boxing manga can live on for 600+ chapters, One Piece can also.