K.Rool hasnt appeared since 64.
People constantly have this misnomer, when there's several games K. Rool has appeared in post-64 that are even on the Wii U eShop. K. Rool was in DK: King of Swing and DK: Jungle Climber on GBA and DS, respectively. He was even playable in King of Swing. He was also playable in DK: Barrel Blast and Mario Super Sluggers. Yes these were spinoffs, but he still appeared regardless.
Of course, Super Sluggers was 10 years ago.
having her an Echo doesn't add anything beyond a character playable.
EXACTLY. That's EXACTLY the point. The only thing Echo Fighters bring is adding a character playable. There's no other reason for them to be playable. That's exactly why Isabelle is so likely. She's perfect as an Echo. She really doesn't have anything new to bring, so just express her personality.
I'm also betting on Shadow and Dixie, both who I think are more likely than Isabelle. Shadow is basically a lock. As for Dixie, Diddy's tail jabs are gone, and Chrom has an altered recovery that's a combination of Roy/Ike. Dixie can totally get her full ponytail whirl recovery now.
As for that image people keep sharing, its really, really cool that it looks a lot like GS. I'd not be shocked if they did a similar image if Isaac gets revealed. But I'm not putting much stock in that.
However, yeah, that isn't a very common camera angle for RPG battles. So it is something of note.