I always thought that after Arthas became the Lich King that the goal was to create an army large enough to defeat all others.
This makes sense given that the Scourge only became so strong because everyone was divided and there was no great counter measure to them. With a huge host with one singular goal, no real infighting or petty politics, then Azeroth would be protected from outside influences like the Burning Legion.
I still wonder what an all out war between the Scourge at full strength and the Burning Legion would look like.