It makes perfect sense to me. If you have never seen an orc before, how could you change your form to mimic one? As a DM, I would rule that the common races of Eberron are an automatic go for shapechanger, but something more exotic like a catfolk might not be. Or if the player wants to shapechange into a race they have heard about but not ever seen, I might require a deception check to see if they pulled it off convincingly.
For me the wording that twigs me is "any humanoid of your size that you have seen."
My reading sees it as you can duplicate
individuals you have seen, but you can't arrange those features in novel ways. So your transformations are limited to duplicating existing people, not forming your own identity. You could look like an only-child prince, but you couldn't look like his long-lost brother (who doesn't exist, it's just someone you invented for the con you're about to run). You could look like the merchant down the street, but you couldn't look like his wife unless you've met her (even if he talks about her all the time). You could look like a
specific human, but you couldn't just look like "a human."
So if you haven't seen, I dunno, a human with long, red hair and freckled skin and deep blue eyes, you couldn't turn into that. But if you've seen one with short red hair, freckled skin, and brown eyes, you could turn into that. And if the only redhead you know has straight hair, you won't be making wavy red hair any time soon. And if all you've ever seen are people with red hair, you could never turn into anyone with brown hair. If you've never seen an old elf, you'd be forbidden from turning into one, even if you've seen a middle-aged elf. And so on.
Admittedly, that might just be a point of clarification that's needed. If that interpretation is the intention, that's making some of my personal favorite changeling characters invalid. If it's not the intention, I suppose a bit of clarifying language would be enough (something like "You can only turn into types of humanoids that you have seen.")
What confused me about the dragonmark rules . . . do you need to take the dragonmark feat three times to gain a greater mark? Or does the mark automatically improve with level? The wording used implied both at different points . . .
My reading takes it to automatically improve with level. Which is another reason why a feat is an "awkward" fit. Not impossible, maybe even the best just a bit...out of place...
How else would you model dragonmarks, if not through feats? If you are playing a campaign that relies on them, you could give all of your players a free feat at 1st level. Or, allow a player to *have* a dragonmark, but without powers manifesting until 4th level when they take the feat.
...
Hmmm, maybe a dragonmarked character could be modeled as a subrace?
A subrace might work (they're tied to races in the fiction anyway, and this would enforce that pretty nicely!). Tieflings have set a precedent for spell abilities that level up with you.
Another possibility might be to treat it like a magic item you can attune to. Start out perhaps with a background, and allow attunement...
A third would be to double-down on the narrative significance of a dragonmark, and treat it as equal parts plot hook and character ability. Though that might not be incompatible with a magic item model...
Anyway, just early thoughts.
Similar problem if you develop warforged feats for different construct bodies. Would a warforged built with the equivalent of plate mail require a feat . . . . or a subrace?
I'd just have warforged *paying* for this stuff. A warforged built with plate mail would have to pay like they are buying a suit of plate mail.