What I am saying here is only theoretical of course.
Before he died, Whitebeard said that when One Piece is found, a war will engulf the world. And that war will be between the Straw Hats and the World Government. What I am thinking is that the great war will be somewhat similar to the final battle in Pirates of the Caribbean: At World's End. Once Luffy finds the One Piece, he will unite pirates and/or enemies of the government against the government.
Here is a list of people of whom will certainly or possibly ally with Luffy:
- the remains of the Whitbeard Pirates and their New World allies
- Boa Hancock and the Kuja Pirates (When the government request Hancock's assistance, she will be like screw you, I love Luffy so I'm siding with him and the government will be like WHAT?!!)
- Jinbe and his faction of the Sun Pirates
- the people of Fishman Island
- the Newkamas from Momoiro Island (if not for Luffy then for Sanji at least and Sanji will be very distressed if he sees them again)
- Coby and Helmeppo (they might defect to Luffy's side and I suppose they can be the Mullroy and Murtogg of the One Piece series)
- Note: In the Pirates of the Caribbean movies, Mullroy and Murtogg were originally marines, but they ended up becoming pirates at the end of "At World's End".
After Luffy completes his journey of finding the One Piece, will he stop adventuring? I hope not, there is always room for a new adventure. The next possible adventure will be: finding the Fountain of Youth. The Kraken and the Flying Dutchman exist in the One Piece world so why not the Fountain of Youth.