So, finally got around to watching and thinking about this, too
First thought: we should not underestimate the logistics on such an event!
I’d say keep it as small as possible, number-of-people-wise and groups-wise as well. I’m thinking 3 GMs and three groups of 3 (or 3-4) people each for starters. That would bring us to 12 people - entirely doable even with only people from this forum, given enough advance notice and careful scheduling.
If it turns out that the 3 GMs cannot be 3 moderators at the same time (for whatever reasons, most likely time issues between running a game and communicating between tables), I’d probably expand the whole thing to 2 moderators, 3 GMs and three groups of 3 (or 3-4) players each. Would still only need 14 people.
I liked the idea from the Watch The Skies! video, that one group would play the antagonists (the Alien Invaders), and I would like to pick up on that because I think that’s awesome. Also, one less job for GMs/moderators, because, you know, an important NPC faction is entirely taken care of this way (who normally would need to be “played” by the GMs/mods, and that would take time and agreement in order to be consistently (and stylishly) pulled off)
So, I’d probably go for three groups, e.g. one table plays the dwarves, one table plays the elves, third table plays the orcs (or whatever, just placeholders really)
Now say, the orcs are invading the country where the others live (but for reasons of their own, there has to be something more than barbarism and pillaging as their motivation…)
The elves wanna repel the orcs at all costs (but cannot do it alone)… while the dwarves wanna keep the orcs away from their own towns/caves/sacredplaces, but couldn’t care less about what they do otherwise… also, they don’t trust the elves (again, with good reasons of their own, ideally)
If a simple setup like that is not enough to create insta tension (which i think it would be, given the very limited info every group would receive, and the natural mistrust against rivals and potential allies alike) and provide lasting investment and things to do (which I think is were it might fall short, e.g. what do we do if there is an alliance between any two of the groups too early in the game?) …
… we can always build in a background event that also takes place, and that is run purely by the GMs/mods, and will develop on its own as long as its ignored by the player teams… and will interact with them from a certain point onwards.
In the fantasy example, e.g. there could be a dragon slowly awakening and stirring underneath that long-dead volcano where it sleeps together with its brood…
So while the orcs invade (and perhaps negotiate with any of the other tables as well, and/or pursue their own hidden agenda), the elves lead counterattacks and negotiate with the dwarves, the dwarves barricade themselves securely in their cave-towns and negotiate with the elves… we could use the dragon to throw a spanner in the works whenever we feel the need to complicate and/or spice things up a bit…
Another thing I like, and would keep, is the fixed turn duration. Maybe even use the half-hour as seen in the video.
This is probably essential to keep all groups up to the same timetable, and will reduce “straying” of any given group (it may be very very easy to get lost in the details of the scenario, and given unlimited time, one group may advance much much slower in their plans than another might)
Then, we need only come up with some hidden connections that none of the players realize (at first), i.e. secrets to reveal in game.
Perhaps the Orc High Shaman knows about the awakening dragons and that is why the invasion was started in the first place. Unfortunately, even the Orc team would find out about this only sometime during the game…
Stuff like this, you’ll get the general idea I think 
And add some board-gamey elements to the whole thing, such as tokens or counters for whatever elements we want them to believe are important when the game sets out
(such as the military tokens, the world map, the Research Tokens, the Nobel Prizes etc. in Watch The Skies!)
This will give groups something concrete to do in the beginning. Also, of course these shouldn’t be entirely useless after all, they should rather yield concrete results and advancements in the game. (It would be unfair to the players to give them e.g. military tokens to play around with, only to reveal later that those all amounted to nothing really anyways…)
But the Real game I think, is not in these. It is in how the teams work together (or around each other) with very limited information about each other’s plans and agendas.
Here is where the fantasy example is probably a bit weak, as its easy to imagine how Orcs, Elves and Dwarves would just stop talking to each other at some point. The United Nations from Watch The Skies! is probably a much stronger setup, as it encourages (and expects, really) that people keep up communications, even while invading each other’s areas of influence, double-crossing each other and so on…
Then again, even in WtS! they had the Alien team, and much, much! less conversation was had with them it seems.
But the others (the human factions) talked a lot, almost constantly in fact. This is a point I feel we should keep in mind when designing the scenario.
…
and yeah, sure, if we get more people together, we can always have 4 tables, or even more… or larger tables with more players each…
but I think, maybe start small, this will be an immense amount of prep and orga anyways, even for “just” 12 or 16 people.
If you guys really wanna go for the 20-to-30-or-something crowd, I’m honestly doubtful if we can pull it off
just sayin’
Opinions? 
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