Mysterious Island would probably have fit just as well as Beastly Kingdom would have. Both Mysterious Island attractions center on imaginary animals.
However, Pandora not only showcases imaginary animals (and ecosystems!), but also emphasizes the conservation message (especially in FoP). Since conservation is a central theme of AK, I would argue that Pandora is actually a substantially better fit than Mysterious Island.
Also, having spent a lot of time in both lands, standing around and staring at everything in awe: from a thematic/design perspective, I think both are absolutely extraordinary, and would have great difficulty trying to decide which is “better” (they each have different/unique strengths).