The wholly immersive land conceit should be the exception, not the rule. Following the success of Harry Potter, there have been many attempts to recreate that magic and varying levels of success.
I understand, and perhaps even echoed similar sentiments like, "Pixar isn't a theme". But the reality is, if you can create a designated area for a certain category of themed attractions, that also works. Simply put, it could be the dark side of Fantasyland.
Good point on the conceit. It can be done well with various IPs just as well as immersive worlds of one.
For Villains, an avenue off in Fantasyland as an expansion there would have made more sense, and I think a better expansion than what we got. I think that a large part of it is, a villain is only as interesting as its hero, and a hero is only as interesting as its villain.
Dark rides without the villain of the story are often not really interesting. Typically there needs to be an antagonist if you are mimicking the environments of the film. This is often even said to be one of the specific weakpoints of The Little Mermaid.
Islands of Adventure Lands that are not Harry Potter are great examples of IPs that had some connections in art direction that worked well for an entire land, plus Jurassic Park and the original custom Lost Continent with sublands for the cultural differences of its legends. I think Islands of Adventure truly did this pretty well. Potter obviously was the hyper specific world location.
A Disney comparison of those lands done well would be Frontierland, Tomorrowland and Toon Town of the 90s. Roger Rabbit was not enough on its own, but there is easily a world from Roger Rabbit where the others could fit in the art direction. Splash integrated into Frontierland very well and Tomorrowland as Buck Rogers future community of good and bad inventions made sense.
Speaking of Tomorrowland, a good example of how well rounded light and dark are within the same land but making sense is Timekeeper and Alien Encounter. Right Across from each other leading to Space Mountain and Astro Orbiter being a beacon of space fantasy thrill. Complimented by the kinetic energy of The Peoplemover as the transit system of this world and to the right, Timekeeper at the Science Center, showcasing technology in a lighthearted inspirational light. Across to the left Alien Encounter, the cautionary tale and dark side of technology when in excess.
It was my favorite land in MK for a long time because of that. Frontierland and Tomorrowland were my favs.