Hey, It's That Voice!: Both the Japanese original and English dub went out of their way to bring back actors from previous seasons. English writer/director Jeff Nimoy specifically asked fans on a message board which actors they would most like to return.
As far as actors that haven't done major Digimon roles go, Marcus is Raiden and Gaomon is Gai.
What Could Have Been: Kouki wound up as a near Complete Monster who most-likely only got away because of the fact that he never actually managed to commit mass-murder anywhere near Kurata's level. However, the original plans for him was to be Not So Different from Masuru, being very similar in many ways but unlucky enough to have been found by Kurata instead of DATS.