I'm something of an insider.
I'm not here to leak any information on the game itself, just a little on why nothing's been revealed:
GameFreak and TPCI consider Legends Z-A their most ambitious game yet and they want everything to be the best it can be. They're going for GOTY. They very much do not want to reveal anything before it's ready, and they figured they'd just spend a while building hype and mystery before they said any more.
They had planned to release a trailer in October that would've showcased the starters, the plot, and a couple of new Megas, but the Teraleak prevented that. They did a good job at seeming calm about it in public, but internally it was super chaotic and they didn't want to do the trailer until they were back on track. There was a plan to put it up in January on the anniversary of XY's initial reveal trailer, but at that point they figured they might as well just wait until Pokemon Day.
The game itself was supposed to release in June but thanks to the Teraleak it's been pushed back to late summer or early fall.