As much as I LOVE the idea of a Dark Kingdom, it just wouldn't make sense as a fifth gate. If Disney announced this new park today, it would still be YEARS away from opening. Not to mention this new love of villains could easily turn the other way before the park is even complete!
I agree. Advocates of the idea usually point out that Disney doesn't have as many attractions to appeal to the teen and young adult crowd. At least, not in comparison to the Universal parks who have far more thrill rides.
But the villains theme itself is very polarizing. Many teens and adults would be turned-off by the mere concept, not to mention the families and young children who would be terrified to simply enter its gates.
It's just not an idea with tremendous upside. If Disney wanted to better cater to the teens and young adults, they could couple thrilling attractions with a more universally-appealing theme.