I still think a more effective way to do a AU universe is the Star Trek Mirror universe rout where all the good people are Evil and all the Evil people are good. I personally think something like that wouldn’t be that out of place actually.
Here’s the problem with that. No one on this show, save for Pan, is evil. And no one is good. They are all delicate shades of Grey. And it would also be way to complicated because A and E are big on “evil isn’t born, it’s made.” So we’d have to see how these character got to be who they are, which is what we’ve been doing for 3 seasons. So they can’t just make Snow White evil without explaining why and that takes flashbackes and backstory. Regina and Rumple aren’t EVIL either, they are people who have had difficult lives and have made some terrible choices. And let’s not forget Snow’s dark heart. So they can’t really do what you’re suggesting, just reversing the roles because no one has a set role.