To me, Union's problems boiled down to "20th level pig farmers."
It showed the big flaw in 3e's treatment of epic levels most vividly: you're supposed to act as if nothing has happened in the last 20 levels. The numbers get bigger, and that's it.
That's....unsatisfying.
I agree completely. But to me, that's "execution," not "concept." Because...
Honestly, I don't think there needs to be an "epic level home base" per se. Part of the pleasures of being epic level is that almost nothing else is. There isn't a city catering to your needs because your needs are very unique in the multiverse.
I agree that epic characters should be among the most powerful, and that the notion of a city that's
entirely epic is silly.
But I think there's a middle ground. I think it does an epic party good if there are at least
some other powerful individuals of equivalent might. Otherwise, the epic PCs are doing
everything important.
Hence my vote for the City of Brass. It's not a city full of epic creatures, but it's
ruled by creatures who are, on average, the equivalent of epic, and it's
visited by other epic-level explorers. And unlike Sigil, it's also got some epic-level challenges built into its very nature and environment.
Now, all
that being said, I'd
love for DMG3--assuming it
is epic-focused, which we're all assuming but nobody has confirmed

--to actually talk about this sort of thing. I'd love to see advice for DMs on making the epic tier more than just the paragon tier with bigger numbers. And I think a "home-base" setting designed around epic levels would be a good way to illustrate some of that.