because of our constant time adventures and playing with different realities, worlds become unstable and merge into one.
1. expansion would require another world revamp - nobody likes fiery, destroyed world ande our most favourite places ruined, ashenvale, barrens etc.
2. Arthas is alive, he is a King and his power matches that of Titans. Turn of events made anduin turn into ghoul and Arthas is the true leader of the Velen's Army of Light.
3. Arthas's sister body merged with Sylvanas and [oh I forgot her name viviene? no, you know this scarlet crusade turned ud brat]. an abominations rules forsaken
4. Arthas is the King of Gilneas - in his world, this was the last bastion of humanity, gilneas line perished and so he became its ruler. Silver Hand was never as strong as it is now.
5. Son of Illidan and Tyrande (yes!) murders Malfurion and marries young Azshara (highborne).
6. Arthas rescues group of trolls and expose them to the Light (alternative for night elves origins), they become a new race, completely new light elves, beings of light and sound (something like angels from diablo but with bodies).
7. Uther is one of Burning Legions generals. serving as the first LK. First one was Thrall and his rotting horde