I'm not going full-blown Archipelago (
http://slatestarcodex.com/2014/06/07/archipelago-and-atomic-...) here. I'm not suggesting that these communities should be like the city-states of old, fully empowered to make local law that "works for them" (however awful) without national law prevailing over it. You can't make a municipal bylaw banning white people from your city; municipal bylaws do not work that way.
More generally, things like human rights are still properly built up and decided by the metaethical process of culture clash, rather than something subjective to any individual culture. Humans are humans before they're members of any individual culture, after all, and there is definitely a core set of things all humans need, whether a given culture believes they do or not. It's in our best interests to keep cultures with different ethics interacting, such that the conversation on such subjects doesn't miss out on anyone's viewpoint.
And I'm also not suggesting that people who are persecuted by the social mores of their culture shouldn't be able to leave their culture—renounce it—and join some other culture. Other countries might not feel strongly on this point, but America was, in fact, founded on immigration of what Sharia law would call "apostates": people fleeing and renouncing cultures that they had irreconcilable differences with. In fact, I'm surprised that America has no specific enshrined protection for apostates, such that violence committed in the name of cultural apostasy would be considered a hate-crime. (I guess it's harder to recognize cultural violence than racial violence, because both the attacker and the victim are of the same race, and the victim hasn't always made any formal renunciation of their origin culture. But that doesn't mean it isn't just as frequent.)
Instead, what I'm specifically talking about here is using intentional communities/culture ghettos to make (proportional, but maybe also FttP?) representative democracy work better. Basically, to eliminate gerrymandering by making ridings into "natural kinds", where it's clear where the boundaries of a riding are. One culture-ghetto = one riding.
If a culture-ghetto has a large enough population, you wouldn't split it to give it multiple representatives, because that would reintroduce the potential for gerrymandering; instead, you'd just either have a riding elect multiple representatives proportional to its size (presumably using positional-choice voting or somesuch); or you'd give have it elect a single representative, but give that representative votes in the House proportional to the size of their constituency.
Consider that, right now, we have the "seeds of" policies like Sharia or white-separatism within every community, because every community has some people who would prefer those things. Every representative has to consider how much their community cares about these issues—and they might even play one side against the other, favoring a minority position if they'll gain more votes from that minority than they'll lose in the rest of their riding. And, besides that, a representative can just end up mistaken about how much their constituency cares about an issue. Or paid off by lobbyists to care about the issue, and then justify caring by pointing to the minority who does. All of these effects mean that we can actually end up with quite a few representatives who are in favor of these minority viewpoints.
Culture-ghettos change this. Everyone from a given culture Foo, within a regional catchment area, will end up living in the same place—and so the same riding. If the Foos (and only the Foos) are in favor of some weird policy Bar, then it'll be very clear to anyone with eyes that the representative of the Foo riding should be supporting Bar (and there's probably political corruption involved if they aren't), and that the representatives of the non-Foo ridings shouldn't be supporting Bar (and there's probably political corruption involved if they are.) You can quickly do a 100-person sample of beliefs of a riding, compare it to the platform of the representative, and use the correlation as a measure of how well they're serving their constituency. Obviously, this makes corruption way, way harder. But it also makes minority positions obviously so. If only the Foo riding like Bar, then only one representative in the House will be voting for Bar.
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Note how this seems actually kind of scary: what if the riding's minority is something like "LGBT people"—probably what you'd see from the core of San Francisco after ghetto-annealing took place. How do ideas like equal rights for gay or trans people bubble up into federal law if minority viewpoints are suppressed? But, note, this is what a House of Commons is supposed to look like; suppressing minorities is what it does by design, by having its members vote blindly for their constituents' beliefs (coherently extrapolated using statesmanship), rather than voting their own conscience.
Thankfully, the complete system is not just the House. The Senate is supposed to be where the people voting their conscience reside, and that's where things like amendments grant new rights come from: Senators recognizing and empathizing with the plight of minorities that have nothing to do with them other than sharing their society.
Right now, the House is just a Senate with very short terms. It's so vague what Congressmen are representing that they're allowed to "play Senator", expressing opinions nearly nobody in their constituency actually has, with little repercussion. With the House this way, the Senate just seems redundant. But, given a properly-functioning House, the Senate is a very important counterbalance: it applies a more technocratic eye to the House's inherently short-term populist moods.
Half the problem of American politics comes down to a system originally designed to play these two forces (short-term populism and long-term political-science debate) against one another, tilting over because one of the two forces is not in play. If the House is busy pretending to be a Senate, the force of populism has no say in legislature. This frustrates the people, and so you get reactive populism inserted wherever it will fit, like the Executive or the Judiciary.
If every Congressman thinks themselves a statesman rather than a proxy for the beliefs of you-and-yours, no Congressman will make law that serves your needs. So, a President ends up elected based on their seeming sameness to their electorate, in the hopes that maybe they can represent their needs when Congress can't; and everyone from local judges to supreme-court justices attempt to rework the law set by Congress in their own hands by pursuing trials only for precedents they want set to create overriding case law they can believe in. The horse is lamed and limping, and a functioning Congress making functional law is the broken leg.