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This is something that I should be posting later, ideally, since you haven't yet gotten to the Avalon World Tour episodes in your ramblings, but I finally decided that I needed to let this out of me soon, so I'm doing so now.
I've noticed, over the years since I discovered "Gargoyles" fandom on the Internet, that many people didn't like the Avalon World Tour for various reasons (the length of time, the absence of Hudson and the trio, the focus on myth and fantasy aspects rather than more "mundane" elements like crime-fighting, etc.). On the other hand (while I may have had my moments of wondering when Goliath, Elisa, Angela and Bronx were going to get back to New York), I quite liked the World Tour. To a certain extent, I'll admit that I'm biased - my tastes naturally run towards fantasy/myth elements. But after doing a little thinking on this one, it increasngly struck me that, aside from all that, something of the nature of the Avalon World Tour was a must for "Gargoyles" at some point.
The reason for this is that the World Tour served a very crucial purpose (besides the general one that you mentioned of expanding the "Gargoyles Universe"). It made it clear that Goliath, his clan, and Demona weren't the only gargoyles left. And that was a crucial step. Because if they really had been, the gargoyle species would have been almost irrevocably doomed to extinction, with only seven members left, only one of those seven a female, and that one estranged from all the rest and very unlikely to reconcile with them. Goliath and the others would have been the "last gargoyles", not only in the sense of being the only ones left, but also in the sense that no new gargoyles would come along after them.
If that had been the case, it would have obviously made a rather depressing series. Admittedly, having the main character be the "very last of his kind" wouldn't necessarily be utterly melancholy - Superman is the very last Kryptonian, and his story's an upbeat one, on the whole. But the situation there's different; Superman's alien origin is treated more as a plot device to explain his abilities, so his being "the last of his kind" doesn't appear quite so melancholy. Goliath and his clan's "gargoyleness", however, was treated in the series from the start as a crucial part of them and their very nature, rather than a similar handy plot device to allow them to serve as effective protectors of New York. And also, it was clear enough from the start that an important part of the series would be the gargoyles seeking to make peace with humanity, to overcome the fear that so many humans view them with. Such a quest would have been futile (in a sense) if they were the last of their kind - the understanding on humanity's part of the true nature of gargoyles would come too late to avert the race's extinction - the best that the gargs would be able to hope for in such a situation was that they might be able to live out their last years without the general human population hunting them down, but still aware that there would be no new gargoyles after them. Not very happy.
So there'd obviously have to be gargoyles living in other parts of the world to ensure a future for the species. And Goliath and his clan would have to come into contact with those other gargoyles for the audience to see that they weren't the last. But the clan's situation would make that tricky. For one thing, there'd be the obvious transportation problems - they can't simply hop aboard the next plane bound for London or Japan. And given how secretive gargoyle clans would obviously have to be in modern times, even if Goliath and Co. had a mundane means of transportation to wherever it was that one of these clans was living, they would certainly not be likely to find out about these other clans easily. The only solution to both questions that wouldn't feel contrived was magic - as in the magic of Avalon that sends you where you need to be. That way, Goliath could be brought to the locations of the clans in London, Guatemala, and Ishimura in a convincing fashion.
So I think that the Avalon World Tour was indeed a practical must for the series, to allow the crucial moment when the clan can learn, as Hudson put it in "The Gathering", "We're not alone. We're not the last."
Hey, pal, I'm with you.
From moment one, we wanted to present an OPTIMISTiC world view, that mirrored Goliath's own. (Not that he hasn't had a bad moment or mood or two.)
The World Tour was a necessity from that stand point for all the reasons you stated.
Plus it was a necessity given some of my future plans. 2198 immediately comes to mind. But there was other stuff too.