I have a party of six dwarves from an existing campaign destined to go into Moria with Balin's expedition... So I'm looking less at Backgrounds 1-6 for each culture, and more at making up our own, and bending TOR mechanics where necessary to fit PCs with their own existing identities.

The first thing I found was that Tóki the Toymaker 'needed' rules-exception Specialities. Skill with fine mechanisms -- adaptable to foiling the locks and traps of ancient vaults -- means Burglary. But his master toymaker mentor also taught him some theory about the Shadow in M-e -- Arda Marred, the dragon-sickness associated with gold, the danger to magicians of being lured into sorcery, etc. -- which maps perfectly to Shadow-Lore.
But...

And there's a Hidden Structure in the Rules that goes beyond this statement. Through orthodox character creation, Specialities marked with an asterisk are available only to Player-Heroes of the corresponding Callings.
So that means a Player-Hero really shouldn't have two Calling-specific Specialities... Apart from game balance considerations, this would run the risk of one person hogging the limelight or cramping the style of other Player-Heroes in the areas in which they expect to shine.
(Admittedly with only 5 Callings so far, any party of 6 will begin to see overlap unless they have multiple Slayers with different Enemy Lores...)

I'll probably opt to back Tóki out of Burglary and let him rely on his Craft skill. But I need to discuss that in another topic!

Cheers,
--Os..

For my first run of The Marsh Bell, Tóki will be Journeying not in search of Balin and Óin but to find his missing mentor, the master toymaker Framleiðandi, who I'd already established as having recently left Dale-town on an unspecified matter of Shadow-lore...

Tóki's Companions won't all be from his previous party of six dwarves, but will include some more orthodox TOR Player-Heroes.

'The Hush', a woodman Warden, was generated at the same time as we were doing Tóki's TOR stats. Since we weren't just choosing from the 6 Dwarven Backgrounds in the book, but were improvising a Background to fit the existing Tóki character, I let The Hush devise her own custom Background too.

She chose the Distinctive Features Elusive/i] and [i]Quick of Hearing. And then I found another Hidden Structure in the Rules.
I noticed that Quick of Hearing is available to every single one of the 6 Backgrounds for Elves of Mirkwood, and not to anyone else (except just two of the 6 Hobbit of the Shire Backgrounds; but not even the 'Fairy Heritage' Woodman). And the example on p. 28 of the LMB is for Caranthir the elf too.
So it emerges that Quick of Hearing is an (almost) exclusively Elvish Trait.

Cheers!
--Os.

Interesting. Please keep posting any further observations like these!

I wouldn't necessarily worry about modifying or crossing the intent of any "Hidden Structures". TOR is an extremely rules-light game. Not without its subtlety, but rules light nonetheless, which means its tolerances are very loose. It's one of the features of a rules-light system. A game system with a thousand moving parts over-engineered for perfect balance (something like 4e), has much tighter tolerances.

You can't really break a game as loose as TOR, so go for it. For example, does everyone in Middle Earth who takes on the role of protector find themselves seduced only by power? Of course not, yet that's also a "hidden structure" if I choose to look for it.

Take a look at the case of Quick of Hearing. If you build a character by RAW, only elves and some hobbits can have the trait. It makes perfect sense according to ME, but it is a de facto rule, not de jure, which is pretty much a given for this type of game. So if you had a Beorning who was really wild and wanted to play up the bear aspect, I don't see a problem with letting him have the trait, or have it only at night, what have you.

In your specific case, I would definitely keep your characters intact even if that meant altering the rules. The history and background of the character should override the specific mechanical expression in the rules if something doesn't 100% fit.

I wouldn't worry about one character stepping on other one's toes, but then again niche protection isn't something I ever worry about in table-top RPGs. Your character should shine based on who they are, what decisions they make and what adversities they have overcome, not because it states in the rules that you are the only special snowflake who gets to do Power-X.