They will die from Old age after the series ends or at the end of the series and here are the scenarios (Please don't hit me after reading this….):
Luffy-As Luffy opens the treasure chest of the One Piece,he is shocked to see that it is a gold colored One Piece bathing suit. He goes insane from the stress of knowing he hunted and fought all that time for a bathing suit and kills himself with one of 4kids patented Hammer Guns.
Sanji-While Sanji's demonstrating his culinary skills at the local high seas nursing home for some starvin' grannies, he tries to pour some wine in a wok and catches himself on fire. poor Sanji suffers 3rd degree burns to his whole body and becomes known as "The Town Freak". After becoming a homelsss alchoholic he is sadly overcame by Cirrhosis of the Liver.
Usopp and Chopper-Usopp tries to get Chopper to transform into some new forms and all are fine and dandy until he changes into a suitcase while Usopp is riding on his back. They are both killed: Chopper by Usopps nose impaling him and Usopp by having every bone in his body crushed by a living suitcase.
Zoro-Zoro becomes a male stripper when he cannot find sucess in opening a Santoryuu Dojo. Unfortunatley though, he tries to ride his swords and not the pole and is sliced in half.
Nami-Trying to score some treaure off a local "Scalawag", Nami tries to seduce him and ends up having sex with him. Little did she know, the dirty sailor had AIDs and she dies a feeble sickly old woman.
Franky-Franky begins to buy extra small speedo's (or as I like to call em' "Man Thongs") and wears one to sleep. He's found dead the next morning from loss of blood circulation.