Either (A) Princess Holy Aura, with the players being the OSC agents on the scene (basically turning On-Site for the Apocalypse into a sort of module), or (B) Adventurer's Academy, with the players being new students at the Academy.
These two had the major advantages of having clear self-contained elements for an intro, and having in one way or another some familiar elements that would make it easy to enter the universe.
Arenaverse was out for a number of reasons; I don't have any of it prepared well as game material (isolated pieces but nothing like a consistent set) and the universe (the pieces of interest) is going to be hard to intro in any reasonable time. The same went for Demons of the Past.
The general Zarathan setting is so broad that it would take days to do a meaningful intro, and doing a quick adventure would probably look MOSTLY like a standard D&D setting, so not as cool. Unless I put the players into Phoenix's actual adventures.
Boundary et al isn't really amenable to RPG exploration.
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