I will be starting a new game, in a world built backwards from every world I've ever done.

Normally, I set up the world structure, and give the players a sheet which details what will be allowed or not allowed, based on the structure. For instance, in my last game, each character had to be the level-0 servant of a traveling adventurer, and they would step up to level one in the first adventure, when their masters would have to give their lives for the sake of the quest that the PCs were to complete.

In my last Champions game, all supers were required to have Silver Age Code and to act like early sixties heroes.

I've just announced that my next game will be different - design the kind of character you want, and then I will design a world that you all fit into.

The game is 2E, and I've told people that if they want a certain kind of character they can have it, even if that means designing a new class. (If somebody wants to play an Elfquest elf, there will be a class with Animal Companion, restricted telepathy, and fighting or magic abilities.)

My question is this: Assuming players who optimize backgrounds well, what pitfalls am I running up against? I've already written the following, to defend against people trying to start off with power they didn't earn through experience:

"So what would you like to play, if you had your own choice? If you want to be a human raised by elves, fine – I’ll find a way. If you want to be the last dwarf left after the century-long Goblin Wars, I’ll establish a century-long Goblin War that wiped out the rest of the dwarves.

To the extent possible, this world’s history will be designed to allow what the players want, not merely what the DM wants. This does not mean that I won’t invent lots of stuff that has nothing to do with you; merely that anything needed for you to play your character will be there.

Of course, there are some limitations. The biggest one is this – you are starting as a first level character. When I say you can choose your own background, that doesn’t mean you can decide your character is the captain of an elite war unit. You can decide your goal is to someday organize and train an elite war unit. But you cannot claim experience. You are starting on your adventures and learning curve.

Your background can include one or two formative incidents. These are intended to provide focus, motivation and persona, not extra experience or abilities. Perhaps your village was over-run by ogres, or your sister was saved by a knight, or you heard an incredible bard as a youngster, and this affected what you want to become. You can use this to establish something your character should have but no longer does – he was waylaid on the road, and now some bandit has your great-grandfather’s sword, perhaps.

You can have a mentor. You can be a knight’s new squire, a wizard’s apprentice, or a priest’s acolyte. Creating such a character will be a joint venture, but this isn’t somebody who will adventure with you, and it isn’t somebody you run. It’s an NPC that I run, in order to put you in interesting and dangerous situations.

You can be part of a group. Maybe you just joined that military unit, but if so, it isn’t an elite unit yet, and you’re the new recruit who cleans out the latrines. Try not to invent a background that prevents you from having individual adventures. The group you’re part of isn’t coming with you on your adventures.

You can invent almost any background, but it won’t give you any extra power, ability, or influence. If you want to be the son of a king, fine, but you are the son of a deposed king. Maybe your goal is to win the kingdom back. It’s possible, but you won’t succeed until you have enough experience to establish yourself as a king. And it won’t be automatic -- the kingship will not automatically belong to the old king’s son. There are no old retainers, immediate or potential, that will automatically follow you. (Maybe you can convince me to allow some potential retainers that will follow you if you can find them, but they will be no easier to come by than any other followers.)"

So what else do I need to include, to allow maximum freedom while disallowing power-grabbing under the guise of background? Assume a wide range of knowledge of the rules, and a wide range of role-playing and min-maxing.

Also, if you knew that the world's history would be written to allow any character background you wanted, what would you do with it? (This second question is asked to allow me to see what kinds of things I might have to invent, and what kinds of problems might come up.)