After some brain storming with my 7 year old son, this is what we came up with...

The pirates and ninjas have fought for centuries, but for the first time peace is on the horizon as you ( a noble ninja ) have fallen in love with a pirate leader's daughter. The unifaction of the families will finally bring peace to piratedom and ninjadom.

But then, things are never that simple are they?

Someone has stolen the blue prints to a new breed of medical robot ( think the Fifth Element ). It turns out that some evil pirates who are against the peace deal, are behind it and decided to rid the world of the ninja clans ( don't they always ). The original blue prints were for building robots that could perform life enhancing surgery, but these evil pirates have tweaked the design so that it turns people who hold the ninja gene into zombies. They build an army of these evil robots and unleash them on ninjadom. Your family and friends etc are now all zombies and you are the last ninja standing.

Your revengfull mission, should you choose to accept it, is to...
1. Avoid getting killed by the zombies ( forgive them for they know not what they do )
2. Avoid killing the zombies as they are your family, friends & brethen ( remember )
3. Destroy the evil robots, without being turned into a zombie
4. Destroy the evil pirates ( except for the daughter pirate of course )
5. Retieve the original blue prints from the pirate den ( traps galore )
6. Find Bob the Builder who's gone into business with JC the carpenter to build you good robots who will restore your zombified blood line back to it's original honour & glory.

Not sure what style of game it would be though. I think it has to be simple and not over ambitious, but fun and full of humour.