Honestly, refluffing the Dragonmarks Subrace character variant options would pretty much work and would be awesome. I think a Mark of the Shadows Shadar-Kai would be a fantastic method or way of thinking about refluff options. Sure the Dragonmark is the subrace, but since Mark of the Shadows is elves only, you can pretty much just flavor it as being a Shadar-Kai that has recieved not the Dragonmark of Shadows, but instead the "Gift of the Raven Queen." Its still an Elf subrace and Shadar-Kai, in this edition, are yet another Subrace of Elves. You can even tweak this even further by allow the Mark of the Shadow's Cunning Intuition feature with allowing the option to replace the Intuition Die bonus to Performance (Charisma) Performance checks with your one choice of any of the Charisma Skills.(of course it you would be stuck to the choice you make in that regard.)
I mean, you all can do whatever you want in your games, and that's fine, but restricting the Dragonmarks to Eberron only, in regard to options, is handicapping character ideas, roleplay, and concepts.
Editor's Note: All final say depends on DM's approval with voter consideration from/and or the cat.