Actually I was thinking of moving away from the human element. Think about playing a dragon. A dragon's desires and wants are rather alien to us. We see gold as a currency, it is bedding to them. They enjoy the visceral screams of their victims and the power they have to wield at their whim. They treat humans the way we treat ants. Fascinating at times but mostly just an annoyance.
Personally, I don't think you really need a motivation beyond feeding or territory to make a story work. Hell, what is the drive behind most heroes in a game. Vengeance. Vengeance is dirt simple. A wrongs B and thus B wants to kick A's ass. The rest of the story is fluff. The motivation is simple.
I think backstory is only as interesting as a passive read. It is the present that should matter in a game. Reasons and motivation are essential but it is the present that you will play in.
Here is an example of what I am talking about using my mutant game idea as an example.
"The Creature finds itself behind bars for years being poked and proded by the odd creatures in white coats that never seem to care about anything beyond something called a blood sample. It finds itself being put to sleep only to find itself sore from where they last cut on it. They subject it to tests and other odd things that only seem to frighten and confuse it, being punished if they are not pleased with the results.
Finally, a miracle happens. A word it can't even truly grasp but it happens. An accident happens while it is being transported and the bars that have been its confirming hell are broken, allowing it into the wild where it first slinks into a sewer.
After a day or so of hiding, it finds its stomach rumbling and stalks upwards towards the fresh night air. From the manhole, its senses are regaled by the scent of meat. Walking, talking meat just like what had hurt it for so long. With a leap and a snap, followed by stifled screams, it wolfs down its first meal, sating a hunger and creating a desire and a need for more.
However, it flees from flashing lights and harsh sirens as more of these odd creatures arrive and mutter about its kill. A new terror has begun but it must be careful lest discovered and destroyed by the hands that made it or the protectors of its new prey. Let alone that it can also smell odd creatures like itself and can only feel hatred for rivals to its new domain."
Hopefully that illustrates the idea that hunger and domain can work as backstory and motivation.
