Copied from my post in Tequila Sunrise's thread
Concept: Law and order for the benefit of the people.
Arcadia is law tempered by goodness, so it's not necessarily the Harmonium philosophy of "good for everyone."
When my friend and I talked about this, we envisioned Arcadia as sort of "anti-Acheron," a land of magnificent kingdoms, each running under their own benevolent codes of law. The kingdoms fought and wrangled politically, but the fighting was always noble and dignified and followed the rules of war. Political wrangling happened, but it was neither vicious nor brutal and always stayed close to the law. Chivalry was held in extremely high esteem and fighting for one's nation, one's king, one's people or even the plane itself was considered the greatest deed achievable on the battlefield. It became a place where philosophers from all nations debated what laws were best for their societies and for other societies, too. It was society first, certainly, but it was different views of what "society first" meant, and all with goodness at heart. Treaties are made, remade, broken, and established once more as the kingdoms and nations constantly shift. People that flaunt the law are expected to face punishment -- not out of spite but because these realms are all founded on the basis of law and they believe that benevolent order -- in whatever flavor -- is the most "good" thing that a person can have.
Of course, "the most good for society" is hotly debated today in the real world, so you can see where a good source of conflict comes from. Some leaders might be more strict than others and while none are cruel (save that for Acheron and Baator), very few nations agree 100% on what it means to govern in a way that is in the people's best interests. One nation might be fascist, another might be communist. One kingdom could be socialist while another is capitalist. One nation could have a hereditary petitioner monarchy that goes back generations while another nation is ruled by an elected Senate. It allows room for all flavors and national styles and each one should have representation. It allows both hot and cold wars. It also opens up a bit more for planar adventures within Arcadia as PCs could get caught up in a war between two Arcadian realms.
Despite the differences, though, it's important to remember that the Arcadians are good people. Rulers are generally LG, and they are LN at worst. The plane's nature makes it impossible or extremely difficult for an evil creature to rise to prominent power similar to how Elysium naturally inhibits evil creatures trying to travel through it. Perhaps Arcadian petitioners have a way of detecting falsehoods and deception that evil creatures perpetrate. Regardless, the leaders are good folk and when evil strikes, the kingdoms are more than capable of uniting and standing as a solid front against the fiendish incursions. This opens up ideas, too, for a war between Arcadia and Acheron. Nothing so prolific as the Blood War, but you have two heavily militaristic planes standing a (figurative) hop, skip, and jump from each other. On the occasions that an army on either plane can break off from its internal struggles long enough to look outward, I think they'd see each other as the next target. Arcadians would see Acheronians as cruel tyrants who use the law to oppress their people instead of protect them (and they'd be mostly right), while Acheronians see Arcadians as weak, simpering pansies too hindered by their own laws to become the fighting force they could be.
Arcadia is not pure LG. It's slightly more lawful than good. Ultimately, my intent is to create an Arcadia that's defined as "Law for the benefit of the people." Mechanus would be "Law for Law's sake." Acheron would be "Law for control of the people." In my Arcadia, the laws governing kingdoms would be more flexible than Mechanus's or Acheron's because the Arcadian laws are designed to help the people flourish, not oppress them. Like I said above, the exact flavor of various Arcadian rulers would vary from realm to realm. One realm might have a benevolent dictatorship while another realm will have an absolute democracy. Ultimately, I think Arcadian rulers would have the willingness to change their rules if it turns out a law is definitively better or worse for the people they rule.
I place emphasis on generally virtuous rulers, but not absolute virtue. To me, Celestia represents the "Seven Virtues" concept while Arcadia is a study in benevolent structure.
In any case, while that idea introduces conflict, to a plane I feel is otherwise one of the duller ones, I think it may fall a bit short of being "planar." Warring kingdoms is nothing new and is quite a common feature on Primes. The fact that the Arcadian kingdoms are universally good and have their own interests as well as their opponents' interests in mind (no burning/pillaging/sacking occurs on Arcadia, for example) may not be enough to define it as a "planar" world. Establishing conflict with Acheron helps (reinforces benevolent law and warfare vs. tyrannical law and warfare), but would those two things really set this idea of Arcadia apart from a "vanilla medieval Prime?"
What I was saying is that emigration/inmigration should be a conflict source in the "life is good" planes, ;P. I don't know, if I have problems where I live, I'd try to go to those "perfect" places and stablish there, on Courage or wathever. But Arcadia doesn't welcome everyone, so I think there is strugle between their Lawfulness and their Goodness. Should an outcast ask for asylum in Arcadia? If not accepted, would it cause reaction against Arcadia? If accepted, I'm sure that planars living there with their not pure souls would cause problems. In my opinion people in Arcadia is stubborn and proud, fanatics in their way. Otherwise, a lot of people would like to go to live there.