It seems to me that the best way to do a campaign world for that game would be to make it up on the fly. Starting with a defined world seems very much against the nature of the game-- by putting down some starting lines, you create boundaries about what does and does not exist within the world (or at least what is considered normal). And certainly boundaries can help someone become more creative because having some of the initial big choices made allows on to focus more on the finer details than focusing on the wider ones, it seems sort of against the nature of the game.
Rather than a campaign world, it might be more important to start with some basic storylines that you want to play through and then build the world as you go. When an element for the world is necessary, make it up on the spot and then write it down for when it comes back later. Players would be a part of this world building-- if someone wants to make a "prince" you can then ask what the prince's family is like and right then and there you know the land your world is taking place in is a monarchy and who the royal family is. But if you already determined what the government was like and the name of the ruler, then the prince concept doesn't make sense.
If a player says they are a robot... you can ask if she is a robot from somewhere else or from around this area and who built her. If she is an alien, then it doesn't necessarily say anything about your game world. But if she says she is manufactured in a factory, then that means you are in a world where commercially available robots exist and they are advanced enough to be sentient. If it is an independent inventor, then you know that this world is one where the technology level is such that it isn't unreasonable for someone to build a sentient robot, though the character is likely either the first time anyone made one so advanced (or, at least as far as anyone knows, it might be the first time).
Basically each time you say something is true about the world, you remove a number of character options from being functional possibilities. It is better to first forge your cast and then start eliminating other possibilities based around that... while realizing that what you make isn't going to necessarily be good when the next group comes up with their character concepts.
