Regiones are tricky things, to my mind. I'm interested in your take on a couple of specific points:
Regiones are different levels of reality stacked on top of each other, right? So, when under the influence of a Regio detection spell, like Piercing the Faerie Veil, what will you see if you are "inside" the borders of a regio but "under" it (that is, at the level of the mundane world)? Will you see nothing, or will you see into the regio but be unable to go there (as it were, like standing on a glass floor looking down on another floor but needing to find a staircase to be able to go there)? Or can you enter a regio from any point "under" it?
Regiones have limited extents (unless you enter Arcadia), but what are those edges like from the inside? If you are not able (through magic or whatnot) to see to the outside, what is the appearance of the regio's surroundings when seen from the inside? Must they be cut off (by, say, a wall or an impenetrable thicket or somesuch)? You cannot, as I understand it, always leave a regio just by walking far enough in one direction, but what happens if you try?
Regiones are areas for the imagination, and as such are pretty vaguely defined- no two may be alike, and any guidelines are general.
One can get trapped inside a Regionne if they stumble into it accidentally- so whether one wanders in circles, or hits walls is of secondary importance.
These are inspired by folk tales, of hunters chasing a red stag and finding themselves in faerie, or walking into a door in a cliff and finding a cave that's only there once a year. Or, later, like Alice in Wonderland- fall down the wrong hole, and you're in hell.
Since they defy the standard 3 dimensions (perhaps time as well, depending), they need not be defined in terms of those. Any and all of your above suggestions would be acceptable. "Predictability" is not a word I'd normally associate with Regiones- the more "magical" they appear, the better for the story.
I'd use your second option (see into the regio but be unable to go there) or only allow the charater to see through the gate ways between levels.
I frequently imagine regios to have borders that loop back upon themselvs (as if they were on the outside of a hypersphere, if that makes sense to you). At the moment however I'm thinking of a computer game that I played in the mid-nintes called arena where you could headout into the woods and go forever..but there was nothing there, just an endless vista of trees hills and wandering monsters the computer kept track of your x,y coordinate but go far enough away from the city and you could be sure that you'd be precisly nowhere.
You dont need the Dark Conspiracy Rules to use this.
Many of the mechanics used for Proto-Dimensions could apply to Regio.
There are a few examples written up.
While written for a Horror Genre game ,
the examples given could easily be adapted for use in Ars Magica.
I had one regio that was a "closed" - it was a marshland centered around an island, and if you went too far out you'd find yourself close to the island, heading towards it, from the same general direction you went out from.
I had (well, not used) another regio that was a whole world. Not arcadia, just a whole world, a different earth if you will.
I agree that every regio is its own thing, and should conform only to its own laws. Generally, I would allow one seeing into a regio to see into it but not be able to pass through to it until he finds an entry. I would make most regios either closed in some way (as above), or allow walking out of the boundaries of the regio to leave it (but not enter it!).
I do not see regios as strictly contained to ever smaller regions. I see them as different levels of reality. A regio may be only accessible from a certain geographical boundary, but often extends beyond it into an area that can be miniscule or can be an entire alternate world.
After reading a lot of myths, faerie stories, and novels, my approach to regiones is ... confused.
One might enter a faerie mound and find a regio -- nice and limited. Equally, stepping into a section of a cave or a forest clearing could be a nicely limited regio.
On the other hand, what of top of a mountain (think of Mount Sinai, obviously a Divine Regio)? Not only can it easily be a regio, it overlooks larger areas. What would be seen? Would others simply see the person in the regio was wandering around in a normal area, or possibly avoiding things "that just aren't there"? Equally, would the person in the regio see the mundane area as mundane, as aura-attuned, or not see them at all?
Then what of a forest where one enters not fully into, say, Faerie, but into a faerie regio. Would this regio necessarily be bound by the limits of the forest itself? Might the area be, much like in the novel Mythago Wood, be more elastic?
I play regiones by ear and by need. Some have limited vistas and areas, others "open out" to a greater or lesser extent. Regiones are in many ways plot devices -- I rarely feel limited by them.
Yeah I've toyed with idea. Over the years I've heard of people discuss Ars (fill in the Blank). I think regios are a perfect way to do this. For example, ene could have Ars Ancient Rome. An entire ancient rome civilzation got 'cut off' from the real Rome as it moved through time. One enters the regio and they are in a past like world- not the true past, but something that emulates it. This idea could be used to allow numerous settings to be explored without breaking Ar's sacred paradigm.
While I suspect sourcebooks to alternative settings will never occur, I've long toyed with the idea of possible one source book that deals with the numerous regio settings that could occur in the game.
A new question: is a regio, in general, affected directly by spells that target the area they occupy? Will a "cold spell" with Boundary target directly affect a regio within the boundary, or only insofar as "correspondence" between the mundande level and the regio dictates?
Perhaps another case of "do as you like" I guess...
My troupe recently decided to set up a new covenant, and one of the virtues we took was a regio. As covenant site storyguide, I then had to populate the regio. Trouble was, I didn't know what was in it yet by the time the magi, half of whom have Second Sight, arrived. So I couldn't let them see into it, but whatever I did had to be consistent with the regio's being there.
I decided that the regio is only 'there' to someone wearing a blindfold. (This hooks up neatly with other magical aspects of the area, which have a theme of perceptual enhancement and moral blindness.) People with Second Sight could tell that there was a regio present, but they couldn't tell anything about its borders or content. If you could use Second Sight while wearing a blindfold (which of course you can't) then you could tell where all the boundaries are, which, as it turns out, is every doorway that's matched by a corresponding dooway in the regio. (Walking through a doorway without a blindfold, or walking around in general, you're just co-located with the regio but don't interact with its contents.)
The various levels of the regio I'm talking about are just indefinite in extent. Perhaps they extend out infinitely. Perhaps you slow down as you leave a central area, so the faster you move the slower you go. Bear in mind that a regio is a volume, not a plane, so if it has edges all around, it won't have a sky any more than it'll have terrestrial boundaries. If you want terrestrial edges but also a sky, it'll have to have a weird conical shape of some kind.
I think I'd have the cold spell only affect the mundane level by default. Keep in mind that a regio can have an aura of different Realm than the lower level; that's pretty separate! Some powerful effects might leak through but in general, yeah, do as you like, or as the story calls for.
I think in general I wouldn't have people seeing into a regio. One of the story ideas is a lake covering both magical and faerie regios, the one with talking fish (Finding Nero the other with people.
Someone said they didn't use the idea of shrinking area as you go up; I note that 5e moved away from that (though it reused the 4e art.) The metaphor now is "building" rather than "hill" or contour map, and in the discussion of a hypothetical Magic Realm a large magical regio is clearly considered possible.
I personally have set up some regio for my saga. I have though set up conditions for entering the regio based upon time of year and specific events which may thin the veil or keep them distinct.
I treat each of the regio as a subset of a larger myth of the area which is onion like because the regio's are variations on how the different realms compete to handle and incorporate the overlarge event.
To be more clear, the covenant in my saga is on the edges of a lake that was created by a asteroid strike.
Taking this event into mythic europe means that litterally a piece of the heavens fell down and struck the earth.
The setting of the covenant is in Sweeden.
I have thus one myth based upon the Monolith builders ( the ones that built things like stonehenge and other stone circles around europe ) and is pre-Roman. It is only remembered by the smallest amount of the community ( old shamans and wise ones ).
I have one myth based on Thor fighting giants. This is the myth that covers post monolith to christian influence.
I have one myth based upon St Michael fighting Satan. This is the latest myth that covers the christian influence.
Each of these four myths layer and create layers of realms and regios around the lake that has the asteroid. Each of the regio's and realms are consistent to one interpretation of the events.
This though is just my way of handling it but I like the onion style feel of layers that give players room to explore these differing views of events and still see an overall connection to one consistent theme.