One piece is divided into four blues (North, South, East, and West), then the Paradise portion of the grand line, and finally the New World. To avoid members creating new characters and Role playing in some of the more dangerous parts of One Piece I've decided to make a sort of milestone for everyone to reach before even attempting to cross over. I'll explain. I'll also add a little description to the oceans.
At first every character MUST RP in the blues. Marines will be stationed there because of their low rank (and nobody wants them) and pirates will be there because.. they suck in the beginning. Yes a character CAN be born in the grand line, NO they can not RP there until they reach a certain level. How does this work for the story? You figure it out. Maybe marines dragged the pirate to the blues, maybe the low ranking marine is stationed in the blues because of their relatively low rank and uselessness. Maybe the pirate went on vacation and his ferry left him in an island in the blues. I don't know, but the character is starting his adventure in the blues. After reaching level 20 a character has two choices; he can either go through the calm belt or over reverse mountain to get to Paradise. Going through reverse mountain, or the calm belt, CAN kill your characters. If you RP your character killing a powerful sea king, or a vice admiral, in the calm belt staff will get involved very quickly depending on the character's level. So RP with realistic events and dangers that you'd think would make the dangers of the grand line and calm belt realistic. Going through the calm belt without issues, or reverse mountain without some broken parts to your ship, is also a red flag unless extremely prepared to avoid problems (such as having sea stone bottoms to ships to avoid seakings and using a different way to sail that doesn't involve wind for the calm belt, or using a ship that is made of metal to get through reverse mountain). Other then that, remember that monsters are very common in the calm belt, so tread with caution.
At level 20 characters are given the option to go to the grand line. So marines have to be captains and pirates have to be.. somebody that doesn't suck. The person doesn't HAVE to go to the paradise portion of the grandline, but they now have the option. I'll make this clear too, a level 20 character is the bare minimum for the grand line. Many things there can be stronger then the character, even at paradise. Shichibukai are lurking around as well as high ranking marines. Dangers in paradise are very real, and it isn't smart to have a decent crew without a navigator, food, and other things important to a crew. A lower level character can only make it to the grand line by joining the crew of a higher level captain that fits the level 20 requirement. It is also something to note that every character, at some point in the grand line, MUST have been in a sky island before heading to the New World. A skilled navigator might be needed for a knock-up stream or it is very likely the crew will lose the ship, or just die. Then again, there are other creative ways to make it to a sky island; like owning a flying ship, for example.
At level forty a character can leave to the New World from the Paradise version of the grand line, Marines MUST be Vice- Admirals. The main difference between Paradise and the New World, is that the New World is at least several times worse. Shichibukai are more commonly found in the beginner areas of the islands, and admirals sometimes come out to kill any unwanted pirates they find (if they have good reason to find them). Marines are more commonly found in this area while in Vice Admiral status, and with big and strong crew members. Marine bases are very big, and the islands that the marines haven't touched are filled with danger and powers never before seen in the prior oceans. Rear Admiral NPC's are almost everywhere, and beasts can take damage or deal damage. It's said that marine captains wouldn't last a day in the New World. It's suicide to go to the New World without a navigator, cook, shipwright, or any of the essentials in a crew for pirates. Unlike the grand line, every character must be level 40 in a crew for all of them to make it to go to the New world. A crew can separate into two halves (or whatever) but everyone that makes it to the New World MUST be level 40. It's also a must for every character to go through either fishman island or marijolas (I recommend fishman island for pirates) before heading to the new world.
Note: To make the staff's job easier every thread must have a title with the ocean the character is located in it. So if I made a thread with one of my characters, it would say something like "North Blue: A Gathering of three" or whatever. The one making the thread can make the title, BUT the ocean has to be in the title somewhere.