I think they made the perfect call with the ones they actually stuck with. Especially Dwight. I mean, I love Seth Rogen, but he would've sucked as Dwight.
Gun to my head, the only two other possible substitutes I think would've possibly worked (or not totally bombed) are the birthday boy (Odenkirk) and Mr. Adam Scott.
But, I'm sure glad they picked the ones we grew to love. An anyway, Eric Stonestreet went on to do great things like be Cam Tucker!