The best way to deal with this situation, I believe, is to just make sure you know the world and its people well, and when the players go to make characters, you be there. So long as you make sure they don't run away with character creation, and make one that would fit in your world, then it should be fine. They need to fit together as a group as well, though I don't subscribe to the: 1 warrior, 1 mage, 1 rogue, & 1 cleric group philosophy. (I started to put that to music in my head as I typed, weird.)
The only other way I can think of is for you to make the characters yourself. The players tell you the generic type of character they want to play in the world & campaign you present to them, but you are in charge of the "nuts & bolts" of the character. Not many would go for this, but it is an option.