I agree that they shouldn’t. Which is why Coco made sense as part of a greater Latin America land with Encanto.
With Encanto clearly out, I could kind of see Santa Cecilia as an extension to Frontierland (even though the U.S. obviously never expended into present-day Mexico), in which case the E-ticket could be lumped in as part of that area. They haven’t built single-IP lands in MK (though I wouldn’t put it past them), but they have done single-IP neighborhoods within lands (such as BatB in NFL, almost Tron in TL).
However, a ride where you fly over the Land of the Dead on the back of an alebrije as a standalone addition to Frontierland doesn’t make a whole lot of sense. You could make the outside work aesthetically, but what connection does the concept have to the land? At that point, why not put Aladdin in Frontierland since it wouldn’t look out of place from the outside?
I’m not the biggest theme stickler and I’m a huge Coco fan, so I certainly wouldn’t complain if they went this route. But I just have trouble accepting that they would decide to move the realistic half of the Coco plans out of Frontierland only to keep the fantasy half in.