I like the idea,The taming idea of the pet NPC was a new twist for me. I know back when you only had SOE and Blizzard as the big boys of MMO's. Both thought about this idea. But it failed to even make it's way into there games. But lets say Xsyon says he would love to do this, But how would it work. Some things are simple. You could make the NPC start off as a kid of 5 or so years old and grows up one year for ever season that goes by till it hits 16 yrs old. At this point it is the minimum age that we can star as. Before it hits the age of 16 it will need food and training. Training would have to be cut short of what we can do because you don't want NPC's being better than your players are. This could add a great time sink into the game.

But here are some stuff off the top of my head that I think needs to be thought about first.
How to limit the amount of NPC's that your players can make. One reason why so many of us are here is that we are sick of dealing with a NPC world. So there would have to be something that makes this hard to do. or takes a long time to do it. And also with some type of cap to how many.
What can you do with your NPC? Can they do jobs? How fast are they at do there job? Does the NPC get to work 24 hours a day, 7 days a week, 52 weeks a year? Can you ask the NPC to do things? Like to go get fire wood, If so, If some one was to attack it. Can it fight back? Can the NPC die? So say it was never attacked, Does the NPC know where to go find fire wood? Does it look for short logs? If no short logs will it find a tree? How does it no when to stop clear cutting all the trees?

But thats just the tip of all the how and whys that would have to be looked at, But I do like the idea, I did a few post and blogs on this a year or so ago. So I would be down for it if they was to put the man hours in on it. Sorry and woman hours on it as well.