Plus, objectively speaking, what exactly you would expect them to reveals us?
They cannot give up too much of the plot of the game, else why play it. The fact that Necrozma essentially infects/takesover the 2 legendaryes is actually a pretty good information storywise which does not give up a lot about the plot, but does tell that in fact there is a parasitary / distruptive thing to be happening, either at Necrozma's itself hands or someone else.
They cannot really reveal new Pokemon, since it's not a new generation, and the Pokedex will be the same minus maybe a mythical or two.
Revealing new moves, event specific / USUM specific pokemon, possibly some key changes to mechanics if present or to the background (like the fact it's not a following story but an alternate reality) is essentially all you can expect in pre-release issues.
It appears we have neither new Pokémon (like in SM and XY) or new formes (Megas in ORAS) that could be gradually revealed over the course of the pre-release period this time. It's a situation we haven't had since BW2. How did they do it back then?
Just for fun, I looked up the CoroCoro issues prior to BW2's release:
February 2012: The games weren't announced yet (that would happen on the 25th), but Keldeo was featured.
March 2012: Kyurem's new forms officially presented. As was a Keldeo TCG card, among other cards.
April 2012: The Kyurem Formes' stats are described. Games revealed to take place two years after the events of BW1. Region map released, showing new locations in Unova. Glimpses of new Unova dex (confirming the return of previous-gen Pokémon). Nate, Rosa (those are the names of the player characters, right?), and Hugh featured in full. Roxie and Marlon introduced (and stated to be Gym Leaders of their respective types). Colress teased. Aspertia City introduced.
May 2012: The Therian formes of the Kami Trio revealed and named. Bianca and Cheren's new jobs introduced. Pokémon World Tournament revealed. PokéStar Studios revealed. Some tidbits on Keldeo and Meloetta revealed, but thanks to data mining we had known that for a while by then.
June 2012: Keldeo Resolute forme revealed. The Kyurem Formes' abilities revealed, and their version exclusivity settled. C-Gear, Poké Transfer, Black City, and White Forest revealed to be back. Info on how to obtain the Therian Formes. Reversal Mountain, Join Avenue, Funfest Missions, and the Medal system teased.
The games were released in Japan on June 23.
So by comparison, USUM news are underperforming heavily. The last three CoroCoro issues before release were
packed with information, at least when compared to USUM. It's true that CoroCoro constituted pretty much all the marketing there was back then, but even if we count the other stuff we've seen, we don't know as much about USUM as we did about BW2 in April 2012. Or at least, we knew that what we knew was only a small portion of all the new stuff. Then again, we have tonight's Direct to look forward to, maybe they will give us more to discuss then.
To reiterate, until now it seems like we have:
- 2 new Necrozma formes, with one known move known in each of their pools.
- 1 new Lycanroc form, and how to obtain it.
- 2 new Z-moves (Lycanroc, Kommo-o).
- 1 new Pikachu hat.
- 1 (possibly event) Pokémon with a new Ability (Own Tempo Rockruff).
- New photo mechanic.
- A barely changed overworld map (artwork only).
- Glimpses of some new locations.
- Glimpses of some old locations.
- "Blank slate" player characters returning.
- Some new items of clothing.
- Some old characters returning (no new ones introduced yet).