Birthright vs Kingmaker, part II
Birthright is generally more about being the Nth regent of an existing realm that's potentially a thousand years old rather than carving a new one from scratch. However, there are parts of the continent that are sparsely inhabited/developed and if the game is set in those kinds of places, then kingdom building can become a bigger focus of the game.
Mythic and Ultimate Campaign
Birthright already has a complete rules set on running a realm. While one could use Mythic and UC to recreate a Birthright style campaign, I don't see any reason to reinvent the wheel.
That being said, like any good GM I consider myself open to shamelessly stealing from good sources. I'd consider those to be potentially on the table at this point in the interest check.
As Dragonofashandflame mentioned, "full" arcane magic is only possible by elf blood or the divine energy of a bloodline. Since I plan for everyone to be the latter (even if noone is the former) Cerilia's unique arcane magic rules should be a non-issue. Realm Magic is going to be much more relevant to the campaign anyway than the "run of the mill" arcane/divine magic common to D&D games.
I don't plan on adapting the setting-specific Magician class; any NPC arcane types will just have levels in Adept, only level-dipped in Wizard/Sorcerer, or simply be blooded scions without a domain of their own. (and thus potentially able to go snatching at unclaimed resources..)
The only arcane magic consideration that's leaping immediately to mind is that Summoners are not thematically appropriate to the setting. Speaking of:
Dragonsofashandflame also identified a potential issue that's particularly applicible to Pathfinder: what classes will be allowed. Birthright was created before multiclassing was what it is now, and the 3rd edition fan conversion didn't properly address it to my satisfaction. What classes can apply divine power to what sorts of domain actions is a big deal in Birthright, and as is there's little disincentive to level dip in all the classes you need to do everything. I'll be coming up with something.
Pathfinder poses a particular challenge: that's a system that is all about providing options and lots of them. But Cerilia isn't the cosmopolitan "we have everything!" setting that Golarion is; lots of things in Pathfinder will have to be banned for a Birthright game. I'm not sure if it'd be shorter to make an "allowed" list or a "banned" list.
Cerilia basically has the following cultures:
If some "thing" isn't appropriate to that culture, it shouldn't be there. By the setting rules, Anuireans can't be Druids and and Rjuriks can't be Paladins, for example. I'm not sure if I want to keep that sort of 2nd edition mindset or not. What do you think?
At any rate, I like Gunslingers and Oriental stuff but those are all utterly foreign to the setting. There won't be any of that, sorry :(
In a birthright campaign, much of the game balance that goes into the classes designed for conventional adventuring is thrown out of whack. Skills and social abilities are far more valuable than normal, as thunderbeard noted in a similar campaign. Another consideration is that in a Birthright campaign your domain is pretty much a surrogate character.. the ins and outs of the thing you rule is often more important than your character sheet. The divine bloodright linking your character to the realm pretty much does make the realm an extension of your character.. often the more important part when the regent isn't adventuring.