I was really happy with Nin, so I'd like to keep her in some form under these changes if possible. I assume there's nothing in her background that's incompatible with the new system or premise, just that she's been gone from home longer and has settled down and become part of this keep of ours, perhaps still as a bit of a wanderer but with strong ties. I'm looking at the Outranger as the most obvious one for her.
Yeah, I think you can keep the same character, and playing her as the Outranger makes a lot of sense. The one thing is that probably Dark Age is a bit easier if she and her people have a bit less overt magic. Less free size-changing and more small people with skillful woodcraft and enchantment that allows them to confuse and mislead (and maybe change sizes with preparation and special commitment to an enchantment).
What system were you using before you made the switch if I may ask? The original Pendragon? or is there another AW hack of it?
As Tatters notes, Fellowship was how we were setting it. Fellowship has a particular structure (originally built to emulate a Lord of the Rings style of story) where there's an Overlord and then a party of united PCs from a variety of different people's opposed to the Overlord. Some of its later books (it has three books you can buy atm) include alternatives to the Overlord: the Horizon (focused on exploration without a single big antagonist) and the Empire (focused on rebellion where the bad guys have already won and control most of the land where the game is played). Setting up a corrupt Uther as the Overlord in a Fellowship game works well, but Fellowship really wants characters with very distinct peoples and lots of high magic and various sorts of technology (e.g., the Dwarf has several explosive and gunpowder options). I had kind of forgotten Dark Age existed and was trying to get this game setup to fit within fellowship, but then realized it was drifting into generic fantasy and away from the Arthurian setup.
I'd say the actual Pendragon system is a bit too clunky to run well over PBP, but it offers a lot of helpful background. The GPC has some particularly useful footnotes and sidebars especially. I don't particularly care for the detailed date/location/name of important person breakdown (too much detail) but sidebars like what types of advisors Uther has at his court and notes on political and geographic intrigue are very helpful for understanding the types of issues people would have been dealing with. The "group of players opposing a corrupt king Uther from a central keep" for Dark Age is a modification of the setup for the default Pendragon campaign.