I can't say I'm surprised that people keep predicting that each book will be "the end of the big three". I remember speculation for that during the NJO, in Dark Nest, for LOTF and for FOTJ.
I doubt that its happening any time soon. Sue has made it clear for years that when that does happen, Lucas has to approve it. And I can't see him suddenly doing that right now. Also, when he does I think it will be dropped in a much bigger bombshell than a simple book anouncement at SDCC.
That said, I'm pretty sure I know where this is headed ...
NOTE: POSSIBLE APOCALYPSE SPOILERS FOLLOW (if you haven't gotten that far yet)
Think about it.
1) Denning is the one who had to figure out how to use that to explain Abeloth and tie FOTJ into the Clone Wars madness. He's the one writing this book.
2) Original Mortis family - the father (wise protector), the daughter (light) and the son (darkish)
The big 3 - Luke (wise jedi master), leia (jedi knight), and han (reformed smugler/fringer)
3) The stuff with Abeloth wasn't completely resolved with parts of her essence showing up all over the place, and I suspect LFL wants to put that to rest.
4) At the end of Apocalypse, Luke sends off a bunch of knights to find the object. That's going to have to be wrapped up at some point.
5) The fan base is divided about Mortis and Abeloth, and it would be a good decision to close both of those out so they're done with and forgotten
6) Without a major galaxy-spanning threat currently in place to be used (One sith must stay hidden, LTOTS would be fractured and trying to rebuild), they need a filler book (or books) until the next series is written. Also gives them more time to plan overarching plots for the next sets of books without the fans getting restless.
To me, that all adds up quite conclusively to this book being about the Mortis object, and something to do with wrapping up that storyline, and (possibly) Abeloth's.
It also makes sense that it would be just the big 3 involved because: Jaina's still in the honeymoon phase of her marriage; Ben is probably licking his wounds from Vestara, the battles of FOTJ and also still being a teen; Tenel Ka and Amelia are going to be sequestered in Hapes for a while; Wynn will be busy getting the GA under control; and the other jedi masters...well, they've never really had big parts lately.
Also, keep in mind, for this to be released in spring, that Denning was probably already working on it long before it was announced, possibly before Apocalypse was released, so it has to be something that was forseeable when they were at that point. And logically that would tie it into the Mortis stuff he had to cram in.
So this makes sense to me both in and out of universe. Far more so than killing off the big 3 all of a sudden in a simple standalone.
That's where I'm placing my bets.