I don't know if Heidfeld is WDC material as he's never really had a real chance of having a go at it..
But I can think of quite a few drivers which Nick would
probably (based on previous performances) easily embarrass with the same equipment (Klien, Yamamoto, Senna, Glock, Trulli, Chandhok, DiGrassi, Kovalainen, Alguersuari, de la Rosa, Buemi, Liuzzi, Hulkenberg, Petrov) and a few others which he would
perhaps beat or match (Kobayashi, Barrichello, Sutil, Massa, Button...and dare I say Schumi?).
That automatically makes him top-10 material. He is also a good qualifier I think, and would not be the one to engage in silly crashes (safety car incident aside!).
Then again, I also used to like Fisichella, Verstappen and Irvine a lot!
The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. H.P.Lovecraft