This is what I picture, and it does have some of the great ideas that some of you have come up with.
The World Government is history. It was taken out by the strawhats and dragon's revolutionaries. There are still marines, but they work for a more peaceful world power that came into power so there wasn't chaos. This world power is lead by Vivi. The leader of the marines is Smoker, with coby and helmeppo high up in rank.
Luffy found one piece, but it wasnt treasure per say, and he's the pirate king. Now he sails the seas for the rest of his life searching for adventure.
Zoro defeated mihawk and became the greatest swordsman. He carries around mihawks black sword as proof. Johnny and yosaku are his heralds, lol.
Sanji found the all blue and became the first person to cook all its fish. He becomes pretty much the best chef. Still travels with luffy, why would he let him go?
Usopp is a warrior of the sea like he dreamed of. He's broken off and started his own pirate crew with 8000 followers (hey, his lies always come true dont they?) kaya travels with him as his doctor. He has giant apprentices. Him and luffy still visit, and even spar, just like shanks and whitebeard. And he's still the same old liar, as well as the spreader of the tales of strawhat.
Chopper has found a cure for every disease and expanded his horizons. World renowned doctor. Still travels with luffy, and sometimes usopp.
Robin has founded a new ohara and big library tree thing, as you people have said. It contains all the poneglyphs. It also contains a map of the world hanging high in its lobby, courtesy of nami. She leaves in it capable hands as she trvaels with luffy.
Nami created above mentioned world map and still travels with luffy.
Franky saw his ship sunny trvael the world. Created usopp's ship, still travels with luffy.
Brook met up with laboon and accomplished whatever dream he had. Still travels immortally with luffy.
Laboon follows luffy and crew around now.
Ace joined up with the best, the strawhats, after whitebeard was defeated by luffy. This is assuming ace is still alive. Aokiji travels with them as well.
Basically all the strong pirates have been defeated by luffy and crew. Blackbeard, shichibukai,whitebeard, shanks, etc.
The starwhats have a large crew, each ship of the fleet created by franky.
Lastly, skypiea has fallen fro the sky and buggy continues to follow luffy to this day.
=] the end