So, it has been mentioned in passing that perhaps there should be an option for Permanent Muto magic. I must admit, this intrigues me, but it would be a somewhat major House Rule if it's implemented. So I thought I would throw it out here for discussion, to see if it would be too unbalancing or an otherwise Really Bad Idea.
4th edition did have a D: Permanent, which was defined as "The spell lasts forever, but remains forever magical. Thus, it could be dispelled at some point in the future. This duration is only available for spells which have magical effects: natural effects are Instant duration instead." The D: Permanent was one Magnitude higher than Year, which would make it +5 Magnitudes.
My idea is this: * That D: Permanent is, by definition, a Ritual spell and thus always requires Vis.
That the effects of a Permanent spell are magical and thus can be dispelled at some point in the future.
That the target of a Permanent spell is subject to Warping (p. 168).
That this Duration be limited to Muto magic. Creo already has something similar (Instant with Vis), and a D: Permanent Perdo would be just hellish. I'm hard-pressed to come up with reasonable uses for Permanent Intellego or Rego magic.
Looks fine to me, just make it a Breakthrough, not a House Rule. If it has "always existed", then you have to rewrite your whole setting to deal with the ramifications. Making it a Breakthrough, your players do the work for you.
I think this works as a breakthrough also, and it would then neatly conform with the 5th edition rules as listed here.
Now then, having said that, many magi (and players) might ask the question, is it worth it? There are disadvantages to this; the vis, that it can be dispelled, and the Warping especially.
Plus there already is a method to make a Muto effect permanent without a breakthrough or a Mystery. An enchanted item can effectively have a permanent effect on its target using the constant duration add ons (2 uses/day + Sun duration + environmental trigger). It uses vis and also would Warp the target and be dispellable.
This shouldn't discourage magi/players from trying to make this breakthrough, but would likely be brought up in any discussion of this new wonder.
It would have to be a Order shaking breakthrough as it is going against the Limit of Essential Nature. That just sets a baseline for the Original Research Breakthrough points. If the troupe agrees it would be worth a shot.
I think it would not be worth it unless it was a weapon because it can cause warping.