As long as its not replicating the way Abyssea explained the roaming and avatars or the KI pops. I can see a fate chain as the most likely plot device. However given the level of the 2.x areas.
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I don't think they'd be in 2.0 areas at this point. Potentially all of the beast tribe areas have room for expansion into their own region, places like Paglth'an for the Amalj'aa (there is a gate at the back of Zahar'ak that could lead right there). Imagine a whole zone in the theme of their strongholds; A much better place to put a wandering Primal. Both thematically and logistically. Stuff them in 2.0 areas and you've got new players seeing Leviathan long before the story introduces him. Have an entire expansion add regions for the beast tribes and you've got the perfect setting.
Interestingly, somewhere like Mount O'Ghomoro (for the Kobolds) was intended as a zone back when concept maps for zones were being made up. Upper and Outer La Noscea were all one map, too, but as you zone in on the left side (around X13Y19) there is a blocked pathway which the concept map labeled as "New Volcano Zone", or something similar. Then you've got that Ixali gate in Coerthas that leads up the mountain, just up from Natalan. Definitely room for them to give us such zones, although Sylphlands might be a bit tricky, and lord knows what they'd do for the Sahagin, I'd imagine they'd just get an island somewhere.
I've been playing since 1.0 beta, and I clearly remember the mention of the concept of roaming primals that FCs could temporarily capture and use.
Also, how can you cite a quote that this concept was indeed talked about at E3, then blame the OP for getting the wrong impression?
This is exactly how I remember it being described, also.