Observation #1: Lack of PJ designs for Tsubasa, Maria, Shirabe and Kirika. Lack of school uniform for Tsubasa.
This likely means that Tsubasa has graduated from Lydian, especially given that G ended around Fall and Japanese school years start in Spring, but it could also mean she's simply taking a leave of absence to travel for her career. Since we know she will be doing another duet with Maria, it seems likely that Maria is with her. By extension, I assume that means Kirika and Shirabe are with Maria, who has always acted as guardian/caretaker for them. This means at the start of the season, we might have the group split up into two.
Observation #2: The Noise are gone. There is no need for the Symphogear anymore.
Things will start out with a false peace. Tsubasa and Maria have progressed their music careers, Hibiki, Chris and Miku still go to school regularly. With the Noise gone, there is no purpose for Div 2 anymore and that may already have been reformed into a new government agency. It's not until the enemy reveals itself that things will go badly, and depending on who is where at that time - it might be hard for them to regroup. Further, the Symphoegar was developed as an Anti-Noise weapon. Now that the Noise are gone, it might not hold up nearly as well against a non-Noise enemy.
Observation #3: America has been a constant enemy.
The American government has always been an antagonist in Symphogear, and I have no doubt that GX will remain the same. In the keywords it was revealed that after G ended the American government pushed for the death penalty for the remaining members of F.I.S. and even wanted to hold their own trial, but this was quashed by the fact that they wanted to hide their role in the whole affair (lying about the trajectory of the moon) and backed off. Later, they presumably killed 2 magazine journalists who published an article that attempted to reveal the truth about the moon. To be frank, it seems suspicious to me that the American government is so overtly concerned about the moon. We know for a fact that the moon is a giant machine left by the Custodians that enforces the Curse of Balal. Assuming Symphogear follows the same history as we do, who landed on the moon first? America. And what exactly did they find out? Maybe more than they let on.
Observation #4: Nastassja rebooted the moon.
Speaking of the Custodians, do they know that we've been fucking with their shit? Nastassja rebooted the moon's systems in order to correct its orbit and save the Earth. The moon was put in place as a way for the Custodians to observe the Lulu Amelu after the took away the Unified Language with the curse. I suspect maybe upon rebooting the moon sent a signal to alert its owners as to the changes.
Observation #5: Frontier, the intergalactic spaceship the Custodians used....was still on Earth.
Of course they probably had other ships and other means of leaving Earth in the past, but I've always thought that this particular plot device meant that indeed some of the Custodians stayed upon the Earth. However, I'd also theorize that their existence might be very similar to Fine's in the sense that they must be "awoken." It's never really explained how Fine achieved her reincarnation system, but assuming she got the idea or borrowed technology from the Custodians...that means they would possess a similar ability. In fact, within actual mythology the figures equivalent to the Custodians were known for toying with genetics/cloning/breeding to create beings.
Now with this all in mind...
Theory #1: The enemy's goal is the destruction of mankind itself.
In G, the antagonist characters fought to save the Earth from the threat of the moon. Despite how misguided Dr. Ver was - his end goal was the same. Prevent the human race from dying out. Kaneko has said multiple times that the enemies in GX will be the opposite. Instead of trying to save us, I'm betting they will be out to destroy us. And if an enemy is out to destroy mankind, then it's not likely to be human.
Theory #2: The enemies are the Custodians. Or descendants of them, who stayed on Earth or came down from the moon in the past.
Kaneko has continually said that the enemy this season will have the ability to hold back the power of the Symphogear. Realistically, I don't think a human force outside of Genjuurou would actually be capable of doing that. So naturally we have to think beyond the boundaries of Earth and consider what if the Custodians returned? Or what if they had been here the whole time? It seems to reason that much of Fine's technological knowledge was based off of what she learned from her society which was in direct contact with the Custodians. I have no doubt that they would easily be able to manipulate/restrain the Symphogear system. However that's not all.
Theory #3: The enemy will have the power to rewrite/take over other people's personalities, and/or clone them.
It seems like one of the main themes of GX will be about fighting your inner darkness, and Kaneko also seemed to hint that the enemy might not necessarily be an outside force, but a force close to Hibiki and co. that is affected by some change. I honestly don't think any characters will die this season, but I think we will be made to think that they did - by the fact that they will no longer be the same person they were. If Fine's soul can overwrite and control her descendants - then what about the Custodians? If mythological references are to be believed - they were responsible for the creation of mankind itself. What greater enemy could there be except for one that can take you out from the inside? Or, alternatively, what if the enemy was yourself? At the very least, I believe the enemy will have control over Hibiki's dark side (or will draw it out of her to use as a weapon) and that she will inevitably be forced to fight against the other girls - an instance that has already occurred both in s1 and s2.
Theory #4: The new enemy will not be susceptible to BEFRIENDING
What can draw out Hibiki's inner darkness? So far it's always been when she was upset over the loss of a friend, or breaking a promise with a friend, or when she herself was injured. Hibiki has never actually gone on the offensive in an attempt to kill anyone...except when she was Berserker. She had no qualms with drawing blood from Tsubasa in s1. Naturally, whatever the enemy is this time I don't think they will be able to work out their problems by talking. This will come as a frustration to Hibiki because it doesn't coincide with her way of doing things. What will she do when faced with an enemy she can't simply talk down?
I enjoy thinking about what may be to come in GX, but if it's anything like G then all of my theories are wrong. But I also feel like we are being given a lot more hints about GX than we were with G. Regardless, in just over a week we may be able to put a face to the new enemy, so we'll find out how far off I was then.