Why not celebrate both sides of Disney Movies. They are children's movies and in some cases the villians in the films just as popular and iconic as the heroes. I think I would actually enjoy that park more than the original Disney theme park. "The Dark Kingdom" could be geared towards a little older audience as well.
It is important for children to understand good versus evil and how the two forces interact with one another. In stories where the "good side" is very clear, but the "dark side" is not, it can be tough for children to understand why the good side is actually good. With a very clear and sometimes exaggerated "dark side", it becomes easier for children to decipher why good is good and evil is bad. In addition, no story is complete without some level of conflict, and without dastardly villains, there would be no conflict.
I think it is important to celebrate darkness in fictional settings for two reasons. Firstly, it should be understood and established that in these situations, the darkness is in fact fictional. Learning to be able to separate fiction from real life is essential, especially in the minds of today's youth. Secondly, I think it is important to understand darkness and appreciate what exists in a world of darkness, for without darkness, there can be no light. If you can grasp an understanding of darkness and evil, you can gain a greater understanding of the importance of good and light.
If darkness couldn't be celebrated in a fictional setting, there shouldn't be Halloween. Americans all love to celebrate Halloween. It is just fun for a little while to pretend to be something that you wouldn't ever be. That is the same logic behind a Disney park for villains. Also, in Disney movies, the villain always loses in the end.
Yes, darkness and evil is vital to every Disney story and sometimes the villains are more memorable or iconic than the hero, but i dont think Walt Disney would have liked the idea of a park centered around evil and darkness. The Disney parks are made for the exact opposite and with the purpose of making a child have a happy and memorable time. Maybe a Dark kingdom theme during Halloween but a whole park would kill Walt Disney's idea of joy, as thats what the villain's are against