imo, a backstory should include a number of loose ends, which the GM can mine for adventure hooks or NPCs. The man who dishonored your sister managed to flee from justice; the last words of your dying father were to beware your twin brother; and a few months ago, a mysterious stranger helped you out with a goodly sum of money, then vanished with a smile.
You can easily add ties to other characters this way. Just include them in some sort of prophecy or warning or message; have them be related to your stepuncles third wife; or bring them a garbled letter from a mad hermit. Include some of the same NPCs or mysteries the other players have been using (Wait, you've been given a golden signet ring by this blonde elf woman, too?)
To give the character depth, add a few goals which are totally irrelevant to the game (not to kill the Orc-king, but rather to visit Aunt Dora when you're in the area).
ime, that's about as far as you can take race/class/level, and it's plenty
