I can't comment on what's going to be in the book, but I really enjoy reading all the speculation!
I think Ancient Magic also suggests that there could be other versions of Eden, too, associated with the different Realms. So you could have a Platonic Eden. I'd love to read what you would do with that!
I like the potential of Eden as a story a lot. Eden is supposed to be a physical place on Mythic Earth, and since God manifested there, I imagine it would be especially holy, with a Divine Aura of 10. Sin also originated there, with the serpent often associated with the Devil, so it probably has a very high Infernal aura as well. And magic may have originated there as well, if we accept the idea that Adam and Eve were the first to receive The Gift. So it might also have a powerful Magic aura. Pity it's buried under water, but that shouldn't stop magi!
One question I've had about Eden in Ars Magica for some time is what happens when a Bjornaer enters? He really is an animal as long as he stays in his heartbeast form. Will the Guardians see this? Will he be drawn to the trees as humans would?
My group has played an extended story quest involving both Eden and the Hesperides. So, yes, you can enter the Garden as long as you take and keep animal form (I used a chimpanzee). The Guardian notice, but do not care (in their view, if you take animal from, you choose to be identified as such). Yes, the trees call, but only if you go nearby, and the animals and the Guardians may warn against that if asked the right questions. My character and another mage managed to stay some seasons in the Garden and became fully fluent into Adamic. 8) 8) 8)
To the sheer envy of the other characters that gave up that part of the quest as "a dangerous goose chase" (OOC temporary real-life calls of the players).
But would there be any difference at all between a Bjornaer magus in heartbeast form and a magus from another house using MuCo(An) to change himself into an animal? Would the Guardians even know notice the Bjornaer Magus and tell him apart from an animal? How strong is the essential nature in this respect?
No, no difference, except that afterward the non-Bjornaer wouldn't change back, while the Bjornaer could still change into his human shape. The guardians let all animals in except for serpents; they don't particularly care about the character's essential nature.
Although we probably won't be playing that plot, I'm thinking of banning them from Eden altogether, on the assumption that the Bjornaer Ritual of Twelve Years is metaphysically equivalent to eating one of the Fruits. Witness how Bjornaer magi do turn into magical beasts eventually. It cannot be a coincidence. Maybe they are stealing into the Garden in spirit through the Ritual and it just takes some time for Divine Justice to catch up with them.
Keeps things simple and might give the House some things to think about.
"Sepents, Sons of Adam, Daughters of Eve and Children of Birna..."