...So far, all good! Based on all of that, I could see (as you do) a "Guide to the D&D Multiverse" or, perhaps, a specific AP that has a short section detailing that campaign world (with sidebars for placing it in FR, or "gating" to it, or whatever).
So, where is the difference? It's in expectations and beliefs. See, here's the thing. Do you know one way that they can ensure that people may not purchase it? If they butcher it. Why would, say, an Eberron fan purchase a Guide to the Multiverse is Keith Baker tweets, "All they did was apply the standard PHB and give us the Eberron names."
There are people out there, right now, that are running, trying to run, or want to run the alternate settings, and they want rules for it. Rules for classes and races and feats that are necessary to that setting. This doesn't mean that a DM can't decide to allow FR-standard races and/or classes (Purple Dragon Knight?) into the setting. But it means that the rules will be there for what the setting *should be* if the DM chooses.
Because, and this is really the important point, nobody is running Dark Sun (to use an example) because they love their standard Elf Paladins. We have a generic D&D setting already. And there is no arbiter of D&D correctness that will keep you from running your Elf Paladin in whatever campaign you want; just those, like me, who will say that we have too many Elves and the only good Paladin is a dead Paladin. But that's okay! I am not the arbiter of campaigns, or your table. You can keep on Elf Paladining to your heart's content.
