next in my wishlist of enchantments from fantasy novels. The Kencyrath series "Dark of the Moon".
The secret base of the Builders is surrounded by a field of "step back" stones, basically you step on a buried paving stone, you are teleported back a few paces to where the paving stone was first enchanted.
This all sounds like an Arcane Connection to a point in space to me.
I am envisioning a Boundary "road" made of paving stones, perhaps 10 paving stones wide. The Boundary is enchanrted with a ReCo effect that teleports 15 paces to the Boundary. Then remove the paving stones in a checkerboard pattern from the road and place them at random locations 10 to 15 paces in from the Boundary. Is how it seems in the book.
In Ars terms you couldn't build a large enough labe to do the enchanting. Could you create a Ritual spell that would cause the above effect?
You can enchant them individually, instilling an Arcane Connection within each, to a place 10-15 paces away.
Once you enchant the first one, it's easier since you have the lab text to use as a guide. But you're gonna need to do a lot of fixing ACs, or have apprentices to do it for you.
What about enchanting a series of such stone slabs, every one teleporting those stepping on them 15 paces into a fixed direction from the slab. Having to deploy the slabs properly should be a small price for not needing ACs.
I don't think you need an AC. It’s a touch range effect with an environmental condition of being stepped on that uses the ReCo 10 guideline of: Transport the target instantly up to 5 paces.
Maybe ReCo Base 15 would be better: Transport the target instantly up to 50 paces.
Also, I don't think the environmental trigger would help, since it's supposed to be a "major magical feature of the environment". But wouldn't the action "stepping on the stone" suffice?
Assuming they are lesser enchantments each stone would require 2 pawns of vis (Base 15, R:Touch). 3 if you need unlimited uses. Pretty expensive, when you have hundreds of them (I'm assuming you'd have hundreds of them).
Using a ritual with boundary I think the simplest would be a non-standard effect to randomly make people in the boundary teleport back to a random place in the road (maybe increase the guideline to 25). Say, for every 10 paces you roll a simple die, on a 10 the person steps on a stone and is teleported back to the road (this is somewhat inspired on the effect of Mists of Change, the stones are cosmetic). Not quite what you want, though.
Easier/most straightforward is to enchant them as ReCo effects, with the trigger being "when stepped on".
Problem is, you've got a lot of stones => It's probably easier to enchant whole sections of the road at once instead of each stone ^^
For a ritual, you may be able to make it work with non-standard targets, but I'm not quite convinced by myself. Anyway, for the sake of argumentation:
For the range, I'd base it on Road, so that'd be +2+1 = +3
Duration is tricky. You'd want something akin to Ring, the type where is targets what is inside the ring only when it's inside (Like a Ring of Warding, where your safety varies when you step in/out). You could probably devise something similar based on Year, so +4+1 = +5
Target... Euh... Ok, let's use Boundary +4. I'm not sure, but serf's parma (and I'm rusty)
So you create your "road" and cast a ritual with ReCo base 15, plus 12 magnitudes = lvl 65.
For a year, anyone who steps on the road is teleported 15 paces "away" from it.
The final effect and game balance is most important in my opinion, rather than how it works.
It would be simple mechanically to create an item that teleports everyone in a circle range 15 steps back. Have a large circle made of granite, so it's hard to break. A terram magi could make that circle given the granite. A bit of vis, no granite needed.
If the desired effect is cool looking but more limited than something else, I'd hope a SG would let it go, and just make it fit.
If the SG goes that way ReCo level 15 moves target up to 50 paces, if you want the item not to make a sound, range sight (+3) duration momentary (+0) target circle (+1). lvl 35.
As it is theoretically target circle, there's meant to be a way to break a circle, so if a stone is lifted, the "circle" is broken. Part of the research involved in enchanting the many stones is the strict instructions on how they are placed.
Lesser enchanted devices don't need to have their capacity opened first. This means that, in the style of the flying castle from Legends of Hermes, rather than surrounding your castle with step back stones, you can surround it with a single, exceedingly large, step back stone, perhaps shaped like a donut. Use some muto terram ceremonial spontaneous spell at duration moon to fit it in your laboratory while you are enchanting it.
You could set the device on the circle that it is targeting (on the inside of the circle, reasonably well hidden) to reduce the range to touch and save two magnitudes.
I would say stepping into the targeted circle would be an environmental trigger rather than a simple triggering action, the device itself isn't even being touched. You would also want to give it unlimited uses per day for 10 levels.
I usually advise not to try to replicate the ways in which enchantments in other games / books work procedurally, and instead try to focus on how enchantments in Ars Magicka work and build from there with the same general intent. You could spend a lab season fixing a stone with an arcane connection to a specific place, then spend an extra season enchanting it to teleport to that arcane connection, multiplied by 100 stones, for 200 seasons of work. Or you could skip the arcane connection fixing, and simply enchant the stones to teleport within 50 yards in a specific direction, and get down to 100 season. Or you could enchant a single item with a constant detect movement (InCo/An) effect, that is linked to an InTe effect to detect if the movement happens where the stones are currently located, that has another linked trigger effect to unleash a ReCoAn teleport spell whenever movement happens on top of a stone. 1 season to open the device, 3 seasons for the enchantments, for a total of 4. Or, as you said, you could research a boundary effect as a ritual, spend vis every year, and possibly bring that research down to 1-3 seasons depending on the magnitude of the effect and your lab total. Trying to abide by the rules of magic in another universe while also abiding with the rules of the Ars Magicka universe is usually a great way to make inefficient enchantments, waste vis, waste seasons.