Well, i like to browse the crossover section all the time, but i rarely find the kind of thing i am looking for: a story which blends in the diferent universes and make one new universe with theyr elements.

This is a chalenge for people to do just that with some of my favourite stories, that is, dragon ball, bleach, fairy tail and one piece.

Now, here are some of the things to consider while writing something like that:

-1) the characters powers: many crossovers worry with who is stronger acording to their respective stories, but that is a mistake that wont lead the story anywhere. Instead since it is a new universe with the elements of all the other universes, you have to see that the characters will change, their stories will be completly diferent, and so their strength will be watever you want it to be so long as it gets the story going.

about the dragon ball power levels, if you read carefully and note the power the characers in one piece at the enies loby saga had where far higher, but raditz who had the power level at most 300 times that of a normal human could destroy a planet while rob lucy who had at least 400 times the power of a human (not counting his devil fruit) could not even come near destroying a country with an attack, let alone a planet. i think that the capabilities of the DBZ characters was greatly exagerated, not even freeza should have have been able to destroy a planet in my opinion, only a continent maybe. you should take this into account when establishing the characters capabilities, dont make them all planet destroyers like this, if something is going to destroy a planet let it be a superweapon of some kind.

Also the best is to not leave the abilityes that where avalible in bleach, one piece, fairy tail and dragon ball as they are, but change them somehow so they will work better together.

Moreover the characters can earn powers that are not from the show they originated from, so as to keep things more interesting, and while i am on this topic, when giving characters new powers make them original, dont just rip them off from other charaters like some people do on their crossovers (like giving the zengetsu to your main charater who is not ichigo).

Also very important is to not make the characters get surpassed and useless like in dragonball (how i hated that) or ichigos friends in bleach, if they need a power up invent a creative way to give it to them, in this sense you should inspire yourself more in one piece and maybe fairy tail,as they are making each remotely main character usefull in the story by giving them fiting oponents to fight and oportunities in which only their abilities will solve a problem. Also dont forget to make the characters surpass each other often to twist the story.

And lastly on the power issue, remember that even if you measure the power of the characters, that cannot determine who is stronger than who, the characters skills and strategy must count just as much if the story is to be fun, (which doesnt mean a character cant win simply because they are stronger).

-2)the universe itself: one piece is known for having a crazy planet for an universe, fairy tail for having a somewhat normal planet for an universe, bleach deals with the diferent planes of afterlife, and dragon ball begins in just the earth and ends with the galaxy as its universe. How to join that?

The first rule is to not be to conservative, you dont have to keep every aspect of theese universes,

my sugestions are to make lufy become a space pirate, the world government be a galactic empire of sorts, the afterlife must be divided in many branches like in dragon ball (just dont make the universe have only four galaxies, please, find another way to divide it up), but have shinigamy and not be a perfect place with weak gods (like dragon ball turned up), also dont forget to make the galaxie or galaxies function in crazy ways like in one piece and have noone with power undisputed, or power everywhere (like frieza did), every superpower must have others who are a possible match for them to generate a state of stalemate (at least temporary).

One thing that i have to ask is to not make super magics or tecnologies that grant all your wishes,

this can really make things way too inconsequnential no mather how many rules there are, and so no time travel, no dragon balls (at least not as we know them), and no easy way for people to return to life (is that realy needed when you added bleach to the story? Cant the character just keep fighting as a ghost?).

-3)the storyline: this is an issue ive had with many stories, they just take the premade arks and only change them minimaly, you have got to do better that that, and here are a few tips:

dont copy the arcs, make new arcs inspired on the events and people from the original arcs.

make new arcs, and not just filler or things which dont really afect the story, remember that you are making a new story everything can affect the end result somehow.

As for the guild fairy tail and the strawhat crew, they may be separate but their characters can join togetter to do things, develop the story so as to make the main characters become tied to each other before they are tied to the guild and make the crew and the guild join forces progressively (the part on how their relations will be is up to you to make up, but it would be fun if at some point they fused completly and ended up traveling on a giant spaceship version of the thousand sunny.

Just because a character is not on the ship with lufy does not mean they are not shown in tha story, make the story follow other people as well making it converge at times and preferably having the diferent fronts maintain contact and usualy strive for the same goal as well as be able to meet when needed.

-4)relations and origins: you dont need to make the characters have the same family and origins they originaly had, some can even be older or younger and play completly diferent roles.