I used search. Inspired by a Brennen.exe post.
We all Know that Garp wanted Luffy to become a Marine. So the question is How high up the ladder do you think he'd get? Here's the Ladder in case you can't remember:
Navy Command Structure
Commissioned Naval Officers
- Fleet Admiral (元帥, Gensui)
- Head of the Marines
- Admiral (大将, Taishō)
- Governors-General of the Navy; only three such positions
- Vice-Admiral (中将, Chūjō)
- Rear Admiral (少将, Shōshō)
- Commodore (准将, Junshō)
- Captain (大佐, Taisa)
- Commander (中佐, Chūsa)
- Lieutenant-Commander (少佐, Shōsa)
- Lieutenant (大尉, Taii)
- Lowest rank seen in command of a vessel
- Lieutenant Junior Grade (中尉, Chūi)
- Ensign (少尉, Shōi)
Infantry and Sailor Division
- Warrant Officer (准尉, Jun'i)
- Master Chief Petty Officer (曹長, Sōchō)
- Chief Petty Officer (軍曹, Gunsō)
- Petty Officer (伍長, Gochō)
- Seaman (一等兵, Ittōhei)
- Seaman Apprentice
- Seaman Recruit (三等兵, Santōhei)
- Chore Boy (雑用, Zatsuyō)
I personally think that Luffy would hit the ceiling at Captain if he was influenced by Shanks. But if he wasn't then he would be a Vice Admiral. His happy-go-lucky pirate attitude would stop the Marines from letting him reach any high than that. But if he was raised by Garp with a respect for the Marines an absolute Justice then he would easily reach Vice Admiral status.
But What about the other Strawhats? Could they handle it in the Marines? Here's my prediction on their positions:
Zoro:Vice Admiral
Nami:Lieutenant-Commander
Usopp:Ensign
Sanji:Vice Admiral
Chopper:Lieutenant
Robin:Captain
Franky:Captain
Brook:Captain? (Not enough info though)