My theorizing was half tounge-in-cheek, but I do appreciate you taking it seriously!
Hey, we don't screw around here! Gotta flex those Dominaria muscles
Oh, and if you want another tidbit about Norse culture on Dominaria: According to one of the
Shadow Mage comics, people in Corondor celebrate Yule. It's only mentioned in passing, though, no real specifics are given or shown.
I mostly just miss when there was inter-planar cultural exchange, and a sense that it was all happening in the same multiverse, not just a theme park where bored, insipid planeswalkers moved between ‘dino world’ or ‘Egypt world’.
Me too, although considering they've systematically destroyed my investment in most of their post-Mending planes over the years and that I don't really care about most of the new planes or characters they introduce, I'm actually glad everything has become so isolated. Having meaningful connections between planes becomes detrimental when most of those planes drag down and potentially ruin everything they come into contact with. Especially with the state of the post-Tarkir, post-Origins continuity. Just give me my Dominaria in a big, safe oldschool-bubble and sprinkle in some Innistrad and Kamigawa once in a while and we're good. Seeing Mercadia and Ulgotha again would be fun, too.
And to address your point again, I feel like it would really help if they just stopped focusing on planeswalkers so much and let the plane-bound natives be the heroes of their own stories. Give me Inga Rune-Eyes and all those badass Viking Berserker Giant Warrior Valkyries, and let the 'walkers die in a fire. Wouldn't make the multiverse more connected, but getting a local perspective with meaningful stakes would make it feel more like a real setting and less like a theme park. Planeswalker cards should have stayed a decidious element like they were intended originally.
And sure, we can focus on 'walkers once in a while, but I agree with what others have said about all of them visiting the same handful of planes all the time. The older novels and even the webcomics had glimpses of completely new planes we'd never heard of and that didn't have to sustain their own card sets. And that was great because it made the multiverse feel bigger and less artificial. In fact, I'd argue it was among the most memorable things from those stories. Barren Gastal, ancient Equilor, the seven planes of Parnash with their infamous dungeons. The Sanctum of Stars that Chandra blows up on Kephalai. The verdant Dyson sphere of Pyrulea. That time Freyalise battled Tevesh Szat over Azoria. That plane with the three blue moons where Xantcha is looking for artefacts, the ice world with ruins of a forgotten civilisation frozen beneath her feet as she and Urza are running from Phyrexian Negators, not to mention their time on Moag. Don't make up an inconsequential new plane for everything, but maybe look outside the Therors-Zendikar-Ravnica triangle and find some middle-ground. And I stand by the idea of a "Multiverse Horizons" style set that I brought up a while ago and that would allow them to feature glimpses of forgotten planes that couldn't sustain their own set. Would work wonders for the IP and its universe.
I do think there’s a weird disconnect in the sudden appearance of Norse cultures and mythology in Terisaire on the onset of the Ice Age that could, at least, be explained by some kind of exchange happening in the Northlands at some point.
I mean, the changes to Terisiare have been explained by migration from the Northland. I don't think the Northland itself needs any further explanation for its culture.