I'd been thinking specifically about orcs. I had the idea that they should be shadowen (I'll just keep using that for now), and started to flesh them out a bit. Or rather, not flesh them out: I thought it'd be cool if on the prime world their heads have the appearance of bare skulls (possibly with a pair of small horns), while in the Shadowfell they're grey-skinned and look more like the Lord of the Rings film orcs from Moria. I also wrote without thinking about it that there are no orcs in the Feywild; as shadowen, this makes sense, of course, and it also fits in with the way that orcs are generally (but not always) distinct from the goblin races in D&D. But today I was thinking some more about their appearance: if they were fleshed out in the Shadowfell but skull-headed in the prime world, how would they appear in the Feywild? I had an image of them as hooded robes containing nothing but shadows, like Tolkien's Ringwraiths, and suddenly got excited about it. Maybe it wasn't just that orcs weren't native to the Feywild, maybe they actually had no substance there and appeared just as shadows—possibly still dangerous and capable of causing harm, possibly not.
The immediate follow-up thought was that, if orcs were just shadows in the Feywild, then perhaps elves were shining spirits in the Shadowfell, likewise insubstantial and possibly ineffectual. It's rather Tolkienesque, but hey, the symmetry is neat. However, it does lead to a potential problem: if player characters can be elves, what happens when they go to a dungeon in the Shadowfell? As I'm leaning toward having the spirit-forms being ineffectual as well as insubstantial, that wouldn't work so well for players. A corollary thought was, hey, why aren't there half-orc options for any of the player characters? Half-orcs are part of classic AD&D, if not old-school red-box basic D&D. (Looks like half-orcs were introduced with AD&D 1st edition.)
So, I have two ideas on handling this concept with player characters. My first idea is that elf characters would have a magic elfstone that anchored them to the Feywild, building off the tether-stone idea and passkey-stone idea I've discussed before. By implication, there would also be orcstones, though perhaps the orcs could be searching for such a thing so that they can assault the Feywild. My second idea, building off my question why there aren't half-orcs, is that there are no elf player characters after all; I would simply substitute "half-elf" for "elf" in any of the class options. Hmm, I thought at least one class had a half-elf option already, but in the current draft none do, so perhaps it was taken out. Half-elves, by virtue of their human side, would maintain their appearance, substance, and effectiveness in the Shadowfell; if I added half-orcs as an option, they would do the same in the Feywild. Now that I've written down these two ideas, I'm inclined to keep both of them: no elves (heh) for player characters, just half-elves, and adding half-orc moves for some classes; and elves and orcs may keep their substance in their opposing plane through an appropriate tether-stone, but perhaps those are extremely rare.
Oh and while I'm thinking of it, I'll just toss this bit out there too: fuck the drow. No demon-spider-worshipping elves in my setting. Really, the proper counterpart to the elves are the goblins; they are, after all, all fey.