Pretty much every post-game Team Plasma member pretty much seems remorseful or blindly following one of the sages (and taken away by Looker). Ghetsis dismissed the Shadow Triad. All the Team Plasma mooks only followed Ghetsis because they believed Pokemon should be free, and after Ghetsis revealed his plan to rule the world in the end, I REALLY doubt there will be any sort of Team Plasma revival. Especially since Ghetsis is a wanted criminal now.
Neo Team Rocket wanted Giovanni back because they still thought of him as their leader. Giovanni never lied to the face of his team nor completely betrayed them like Ghetsis did. Giovanni simply walked away.
There was an Ex-Team Rocket Grunt living in Icirrus City during the winter. (The American Grunt that spoke terrible Japanese or w/e language of the translated version you have in G/S/C/HG/SS that stole Machine Parts from the Power Plant) Team Rocket probably did at one time have an Unova branch so I wouldn't be surprised if Team Rocket was being revived in Unova. It's
extremely unlikely, but it's far more likely than Team Plasma ever existing again.
Albert Wesker & Umbrella are more likely to show up than Team Plasma is.
But going back to this picture:
They're standing a few steps up from here:
They're blocking the way to The Little Dome & The Big Dome and/or Route 5. But even though you
should have came from Route 5 because we started in the West, notice how the Rival event seems to happen on the opposite side of where you
should have came into the city.
That heavily implies that you'll be coming up through Route 4, which tells me that the Under Water Tunnel in the trailer may very well be a way from South-West Unova to Castelia City, which would make since as that would make Burgh & Elesa the 3rd & 4th Gym Leaders just like in BW1. (assuming Homika & Shizui are the 1st & 2nd)
But for some reason they're blocking off the Sports Domes instead of the way to Route 5. Which implies something of importance is there.
And it's not this:
Because that building is right in front of a rock wall in a place that doesn't fit anywhere in current Unova, and certainly not Nimbasa City.
I have a sneaking suspicion that the place where you & Akuroma are walking on-to a stage is what they're blocking off, replacing one of the Sports Domes. (The place in the picture above is a movie studio, not where you see Akuroma. There's a quick shot of the movie set inside in the BW2 Trailer where you see the overworld sprite for "Mecha-Tyranitar.") Which would make a lot more sense than blocking off the Sports Domes of all things. But even that would beg the question "why are they blocking it in the first place?". Blocking a building's one thing, but they're blocking (assuming there's 4 more on the other side) nearly HALF THE CITY! Not even the Team Rocket takeover of Saffron City blocked that much of an area off.
But that's all speculation~