I can't imagine the OP life action even having the budget to do marineford. Let alone that if by some miracle the story makes it that far and it's still good the cast will be 20 years older. At which point they would have moved onto other things or just have to be recasted simply because they are too old to look the part. At this point in the story luffy should be 17, zoro and sanji 19-20, nami 17, chopper 15 I think... As great as the casting generally is (though I think the zoro actor is incapable of capturing zoro's goffy or badass side) you'd think them 20 years older than they are supposed to be won't look good.