The opening animation seems to drop several hints of the game's original story, including the appearance of a new relic which I will expand on later. The game itself seems set-up like any other Japanese mobile game. The home menu options include: "menu, notifications, present box, quests, versus arena, card collection, music box, missions, shop, gacha and the commander's assist box." There are 3 types of "currency" in game: energy (which is required to play) in the demo this was maxed at 999 and single missions seemed to take 3 energy, with a 3 minute refill period for 1 energy. The 2nd type of currency are coins, these will likely be used when you strengthen a song or character. The last type of currency is of course the zessho stone which is needed to use the gacha.
The gameplay consists of multiple parts. The first part is the "adventure mode" where the story is told through character portraits and dialog. The demo consisted of the story from episode 1, season 1. The character portraits are not animated and only a small fraction of the dialog is voiced. Sometimes there are brief cut scenes taken from the anime.
The second part of gameplay is of course the battles. These happen in waves where you mainly fight smaller enemies, like the Noise, until you reach the boss. In the demo Nehushtan Fine and final form Fine were seen as bosses. The gameplay is an active time battle, your characters will attack and move on their own but the player will have control over 2 important functions: final attack moves and singer changes.
There are 2 types of final attack moves: physical and special (projectile). On the bottom right of the screen each character has 2 different attacks than can be used when charged. The yellow button is for a physical move and the purple button is for a special move. After use use a move, it can be used again after a cool-down period. If you tap the character portraits themselves, you can change the active singer and song. Songs give buffs for 5 turns before you can change the song again. After you win a battle you'll be treated to a voiced character victory quote, which will change depending on who is in your party. For example at the demo: Hibiki - "Kanade-San! Let's finish with a cool pose!"
For character selection, you can choose up to 3 playable characters. In the demo this included Hibiki, Tsubasa, Chris, Shirabe, Kirika, Maria and Kanade. Miku and Serena were not playable but will be in the final version of the game. Each character has a different type: Strength (力) Intelligence (知) Physical (体) Technique (技) and Spirit (心). This will likely be important in battles as certain types will have strengths/weaknesses over others. Character's gear appearance can be mixed based on season, one group included season 3 Hibiki, season 2 Maria and season 1 Kanade.
Each character you select comes auto equipped with a song, which you can change in the music box. Each song has a buff that boosts the characters. For example, in the demo you could use Kanade with her new original song "Backlit Resolve" which boosts all "technique" character's attack by 1% at level 1 (there are a max of 40 levels). Each song is apparently the full version lasting about 3 minutes.
As previously announced the game will feature original moves, the demo included a new move from Fine called "Imperial Edge" as well as Hibiki's new move: "Garyuu Gekisou Retsuha" (Self-Taught: Striking Spear Extreme Blow) which you can check out in the video below. The video also demonstrates that each card will have 4 levels of "awakening" needed to get them to max (and change the character art).
Unseen in the demo, but heavily featured in the promotional material are the "Memoria Cards" which seem like they will only be used to unlock the "what-if" story scenarios that are playable in the game. It's unclear if they will provide any sort of character buffs.
Lastly, let's take a closer look at the opening animation. It seems to take place after GX based on the gear designs and characters who appear (notably Tsubasa and Hibiki's fathers). However, it also shows glimpses of all the characters from past seasons, but only after the appearance of a new mysterious object.
So, what exactly is this? There are a few theories going around already. Based on the shell like design it might be the Greek God Triton's conch, or it could be the Shankha of Hindu origin. However, it most likely is Gjallarhorn, the horn from Norse mythology. Interestingly, Gjallarhorn was used by the god Heimdallr to awaken and summon the Gods prior to Ragnarok. Heimdallr is also known for bringing dead warriors (known as Einherjar) from Valhalla in order to kill giants and other enemies. This gives us a possible look into XD's story: the resurrection of past characters to fight past enemies, or perhaps a merging of different timelines due to the influence of this new relic. However it's unclear who is using it, or why, or how at all this will tie into AXZ.
In any case, the opening gives us an exciting look at what it will be like to mix and match characters from different seasons, and also gives us a glimpse of how awesome it might be if we can call on certain characters for support.