At one point, Link insults the Elder Yeti by saying that her accent is weird. While this could easily be interpreted as him talking about her inexplicable British accent, the brilliance comes when one recognizes he's referring to her roaring the true name of Shangri-la.
The Elder Yeti and her Yetis aren't simply cold, they're inhumanly cold, and (as a certain Monkey would bitterly put it) "perfect" because it's their nature as, well, yetis. This makes them a foil for Mr. Link, who is warm and humane, despite not being human.
Their color scheme is also symbolic. The yetis have blue skin and white fur to reflect their cold nature, whereas Mr. Link has pink skin and auburn fur, matching his warm and kind personality.
Given this movie came out roughly around Easter time, it shouldn't be a surprise Laika added someEasterEggs into the movie.
As the fifth film Laika has made, the movie appropriately visits (more-or-less) every part of the world the past four Laika movies have ever taken place in. Lionel comes from Britain, finds the Missing Link in the Pacific Northwest, they and Adelina take a cruise from the East Coast, and go to Asia to find Shangri-La. (Bonus points for Britain also being the setting for another joint-Laika film Corpse Bride.)
Mr. Link is pretty lucky that Adelina recognizes how wrong it is for Sir Lionel to treat him like a servant rather than a person. Part of the reason could be that, as a woman at the turn of the 20th century, she understands what it means to be part of a discriminated demographic, not to mention she had a husband who treated her as a equal.
Sir Lionel must've inherited a knighthood; that should make him eligible or even above the Optimates Club. ...Except the Optimates are Lords, at least one rank higher than a knight.
Sir Lionel showing off to Stenk about the existence of Mr. Link may be a stupid move (since it gives Stenk all the more reason to pursue them), but that just may be the point! It's meant to establish Sir Lionel's Fatal Flaw, his need to share proof in order to validate his self-worth and feed his ego.
Gamu claims that Shangri-la isn't lost, but hidden, and that it is the men who search for it who are lost. Perhaps she does recognize that Sir Lionel is lost in his own way.
During the climax, there's a notable Rule of Symbolism on the bridge to Shangri-la. In the standoff between the good guys and bad guys, notice who is standing across from who. Susan is standing across from Stenk, the former being human despite being a sasquatch, and the latter being inhuman despite taking a human shape. Adelina stands across from Mr. Collick. Her strong spirit and tendency to disagree with Sir Lionel's selfish behavior is contrasted with Mr. Collick's weak will and habit of playing sycophant to Piggot-Dunceby. The keystone to this symbolism is Sir Lionel vs. Piggot-Dunceby. Their conversation about shaping the world and acceptance is the culmination of Sir Lionel facing his reflection and overcoming his selfishness. In short, everybody there is literal opposites.
The movie opens by showing Sir Lionel failing to capture photographic evidence of a cryptid, the Loch Ness monster, and ends with a photo of him and Susan (another cryptid).
Building on that, the movie began with Sir Lionel disparaging photography as being inadequate, much like how Lord Piggot-Dunceby looks down on electricity. This may unwittingly reflect that had Sir Lionel not gone on the very adventure that changed his perspective on what's important, he would've been well on his way to being just as obstinate as his adversary. The movie ending with a photo of Susan and Sir Lionel is meant to imply that the latter has come to embrace new technology rather than write it off as inefficient, almost as though signifying that he's taken a different path of openmindedness and humility, unlike Lord Piggot-Dunceby.
It shouldn't be much of a surprise that this movie's Lighter and Softer tone compared to previous Laika works also features a protagonist who isn't a Kid Hero. Coraline, Norman, Eggs, and Kubo are all children with not that much worldly experience and weren't exactly prepared to deal with the perils they faced in their respective movies. Sir Lionel, on the other hand, has been in the adventuring game for quite some time by the start of his own movie, and is obviously more accustomed to the kind of danger he experiences in his line of work.
How did Stenk get those scars on his head? They look like claw marks from some really huge beast, and considering that he's an Egomaniac Hunter, it could have been a hunting accident...
Susan has finally found his place among real friends, but it doesn't change the fact that eventually, he will pass on, leaving the Sasquatch extinct.
How many other innocent travelers came to Shangri-la only to be imprisoned for the rest of their natural lives?
How exactly did the extremely isolationist yetis even know anything about the outside world, much less about their "redneck cousins" from a world away, much less know enough to form that opinion of them? While they could have just kept exceptional records dating to a time before their isolation, it's just as likely (and twice as horrifying) that the Sasquatch may have tried to send Ambassadors to their cousins over the years. Ambassadors that were treated just like any other outsider and kept prisoner for the rest of their lives, indirectly contributing to their shrinking numbers.
Who's to say that the avalanche that killed Aldous wasn't triggered by the yetis in order to silence him?
Despite Sir Piggot-Dunceby trying to kill Susan and Sir Lionel Frost before they reach London in order to stop them from proving the missing link exists (proving Frost right and allowing him entry into his club), Frost never stops there, despite arriving at least within the area on a row boat. While he did promise to bring Susan to his relatives, he could have done so after proving he was right, as it is on the way...
He was probably convinced by his ego that while he could easily stop in London to show off Susan and prove Piggot-Dunceby wrong, it would be far more satisfying for him to bring back evidence of an entire tribe of creatures related to Susan so he could rub it in to his rival's face even more.
