Invoking more thread necromancy here, but it seemed better to contribute to a discussion among others who were once interested rather than simply creating a new thread.
This is all on the subject of 'Mythic Europe' all being a big 'Regione +1' over a mundane world that characters do not usually see. It may take a few pages.
First of all, I liked the Realm of Reason in 3rd edition. It was cut for various reasons; some of them quite good (copyright law, being too early in the timeline, etc). But Mythic Europe is a place of stories, and one of the most compelling stories is the (TORG rpg-like) struggles of various supernatural realms (Divine, Faerie, Infernal, Magical, Reason, etc) to accrue influence over the hearts and minds of people who are just trying to make it through their daily lives. It is more than just a story of thieves and kings; it is a story of gods, too. Reality is reshaped by who those who carry the day.
But this line of (admittedly unfashionable, 'Belief Defines Reality', Creature's Perogative) thinking leads me to propose also the Realm of the Common. It is a Realm which has no powers, but interacts with powers from all other Realms the same way (D: -0.5xAura, F: -0.5xAura, I: -0.5xAura, M: -0.5xAura, etc) -- primarily it is simply the original state of the world, and simply acts as an impediment to establishing any other type of Aura. Other Realms are also welcome, the more the merrier -- provided it doesn't swamp the Story Guide.
I mentioned another rpg (TORG); and I will do it again: "The Primal Order". TPO was one of the very first RPG products by Wizards of the Coast, although they no longer seem to admit to ever having printed it, and a few supplements which were promised never made it to print. The Primal Order was concerned with playing as gods, and described a cosmology which made gods limited and interesting; they vied for control over 'Planes of Existence' and 'Worshipers' because these things enhanced their power which otherwise grew only very slowly. Gods and planes of existence were described with very similar mechanics as well; and worshipers of a god could (upon death) be absorbed into the god -- whose body presented an appropriate afterlife (ie, the 'body' of the god essentially was geography which was only accessible to the souls of worshipers; and populating it also gave the god a little boost of power).
Ars Magica doesn't do Planes of Existence, though. But Regiones are almost exactly 'Planes', and 'Networks of Regiones' leads me to a thought experiment. Here it is:
Make a saga in Mythic Europe unmappable.
Most commoners never leave the village they were born in; beyond the edges of the fields, off the edges of the roads, outside the tiny circle of light cast by human civilization, lurks the Primeval Wild. A typical villager (not trained to survive in the Wild), children, or so on -- if they wander outside of their village, they enter the Primeval Forest and become lost; they wander until they starve, are killed by Fierce Beasts, or otherwise die -- unless they are extremely lucky. The Primeval Forest is infinite in all directions; it cannot be mapped; it changes constantly; and it is at least mildly hostile. Similarly, the Primeval Desert is absolutely infinite in all directions; so is the Primeval Grassland, and the Primeval Ocean, the Primeval Mountains, and so on. The only safe way to traverse Primeval Wild places is via a 'path'; a road; a known, pre-charted course; and so on. The paths have a known (approximate) travel time, but no actual meaningful direction or location.
Very few, hardy people posses the skills to survive in the Wild. Mapping the Primeval Wild is futile; it is simply a list (which is quite easy to modify or update on the fly) of random encounters. Nobody ever settles in the Primeval Wild -- they just create a new settlement which is (of course) surrounded by the Primeval Wild. Ruins of abandoned (forgotten, or destroyed, or otherwise depopulated) settlements (and castles! and temples! and tombs!) can be found in the Primeval Wild -- and lost again, if the explorer perishes or simply walks away. There needs to be a method for certain hardy (specially trained or skilled) folk to create, discover, or 'rediscover' paths through the Primeval Wild -- I do not know what that method is, but it needs to be dauntingly non-trivial.
'Paths' connect the places we are actually interested in -- Planes of Existence. Or, Regiones. Or, settlements of some sort. Each settlement connects (via paths) to one or more others; and their precise geographical relationship is unimportant. Each settlement, each area of civilization and the worked land around it, has an aura and may (or may not) be a Regione. Imagine two villages connected by a three-day path through the Primeval Forest; over a year, each village grows and clears an additional days travel worth of land around each of them -- the path through the Primeval Forest is still a three day path; the Wild cannot be cleared or shrunk, it is always infinite.
In some cases there may be NO POSSIBLE geographic mapping between settlements. This is fine -- magnetic compasses were not a feature of Medieval Europe, and even if they had been only a very tiny minority would know what they were, know how to use them, or be able to afford them; and celestial observations (which depend on a clear view of the sky and the horizon) were similarly difficult. A trip to the big city was not a matter of 'take the interstate north for this exact distance', it was more a matter of 'follow this guide who has been there before' or 'get to a place where you can find a guide'. Maps (where they exist at all) will be 'strip maps', turn by turn directions, not depictions of the geography. This is a place where Mythic Europe can be wilder and more interesting (and more about cool stories) than Medieval Europe. Each location is now a separate reality; sometimes with different rules that govern the reality -- and the actions of characters can influence not just the events in a specific story, but also aspects of the location. Maybe a village will become more or less aligned to the Fae as a result of player choices -- or the Divine, Infernal, or Magical Realms, or other realms you may introduce.
Anyhow, that my two cents. And I realize that Primeval Coastline, and Primeval Rivers might require special handling -- and there are other aspects which need to be fleshed out before this is playable.