Homebrew Campaign setting: The Half-Remembered Realms

TL:DR version-- a mix of the computer game "Majesty" and the novel "The Once and Future King" by T.H. White.

In "OaFK", the real kings of history (Richard I, Edward I, etc.) are considered legendary, while the reigns of King Uther and King Arthur are history. Uther replaces William of Normandy in the novel as "the Conqueror", and rules from A.D. 1066 until his death in A.D. 1215. Arthur pulls Excalibur from the stone in that year, proving he is the rightful king, and reigns until 1485. The Battle of Bosworth Fields is fought between Kinng Arthur and his nephew Mordred.

There is a lot of potential here, both in Uther's reign, which is a time of Force Major--"Might makes Right"--and Arthur's, which focuses on his mission to end Force Major. He establishes the Order of the Round Table to accomplish this goal, but in the HRR, he also reorganizes the informal Adventurers' Guild system that existed since before Uther's time, in order to assist.

The Normans in this campaign setting are represented by Eladrin (Arthur and Lancelot and Guinevere and the KotRT are all Eladrin). Elves are the Welsh, Dwarves are the Anglo-Saxons, and Halflings are the Irish and Scottish. Humans are a mix of all races, except Norman. Dragonborn are the soldiers and servants of the Eladrin-Normans.

As in "Majesty", nobles build different guild houses (Fighters' Guild, Wizards' Guild) to train up adventurers to battle monsters and bandits.

Might I suggest you set this explicitly in a “Feywild”-like realm with the mundane world as its mirror.

One of the many appeals of your source material is the mundane folk who step through into the magic realm, often by accident, and must navigate the magical world to return home (or sometimes choose to stay). This would be a more interesting place to slot the humans into the setting; they’re the rare folk who step through... half-elves are the children of thise who either choose to stay or at least took a lover while there.

Another place where you could take advantage of 4E’s mechanics is that because normal weapons deal untyped damage and almost nothing resists or is immune to untyped damage... which sounds exactly like what “cold iron” should be relative to the fey realms.

So here’s the suggestion; all feywild weapons have a damage type; fire, cold, radiant, lighting, necrotic, etc. ONLY weapons brought from the mundane world deal untyped damage.

Here’s potential irony for the setting... Excalibur is actually a mundane weapon pulled into the fey world by the Lady of the Lake. In fact, take a cue from the sword of Captain Carrot in Sir Terry Pratchett’s Discworld series that is SO MUNDANE it is basically the platonic ideal of a sword; Instead of some impossibly ornate weapon that gleams, it stands out in the fey world by being a simple blade with a basic cross guard and pommel. Wielding it doesn’t give you a feeling of power, it gives you eventual callouses. It’s power is simple... it’s so real, so mundane that it can cut through anything that’s unreal or magical and THAT makes it the most dangerous weapon in the fey world.

Those are a few thoughts based on your initial descriptions. Use or discard as you will.