AG-Congratulations on motherhood! I think that both your plastic surgeon and my dermatologist had the same thing in mind. . . to cut out the scar and resew it. However, maybe your plastic surgeon didn't understand what you hate about the scar- that's it's depressed- an if you're like me, that it looks just like what it is, an old piercing. Even if he left you with a bigger, horizontal scar, the scar might look better- more like a line that can fade, instead of a dark hole!
Did your plastic surgeon mention lasers? Even if you get the depressed area resewn and it leaves a bigger scar, it still might be a more natural looking scar, and it also might respond better than a depressed scar to lasers. I know this might all sound extreme, but I know I think anything would be worth fixing this thing!
Sorry this is so long, but I hate bugging my friends about this, and it sounds like you loath your belly button pierce scar as much as I do! I think you should keep looking for a derm/doctor who understands you truly hate your scar, and that while the scar might never "disappear", it can still be a better looking scar than it is. I think a lot of people assume it might not be worth all the trouble because it's "small." I also think the plastic surgeon gave bad advice when he suggested repiercing it! Write back when you get a chance. . .