I don't understand the complaint.
Are any of these even close, or have I totally misunderstood the problem?
The problem I have with it all is that walkers that don't leave their home planes also don't demonstrate anything that they didn't learn elsewhere either. The reason you make someone a walker is for the experience they can get that planebound characters can't, but what about Daretti, for example, makes him stand out from Grenzo?
Nothing.
Except legs.
He has no skill or viewpoint he picked up from another plane, his time spent off world wasn't meaningful. It doesn't demonstrate any growth that being a walker was
necessary to.
Now, with Daretti, I'll give you that's all of one story and makes a terrible sample size, but it's more a symptom of a common theme in a chain of walkers.
Narset has nothing to distinguish her from the other Jeskai other than a philosophy. That she learned on Tarkir, not on another world. Being a walker isn't utilized for her character at all.
Ral is pretty much the posterchild of an Izzet guildmage. Don't you think he could catapult himself higher if he picked up some foreign magic/tech? The Izzet aren't the pinnacle of technology. (This one at least has a conflict that he's trying to keep it secret, which means they're using him being a walker for something. But if he never goes anywhere, it makes it hard to justify his paranoia.)
Vraska, on the other hand, has no skill or method of thinking that couldn't be learned on Ravnica. She's outside the guild structure, but she doesn't appreciably
feel like she's been anywhere.
Domri, as much as I hate the turd, feels like he's changed because of his travel. Even if he was gone for literally ten minutes.
Now, this isn't a problem with all native walkers. Arlinn might not have any reason to leave Innistrad now, but she learned a unique skill off world. Xenagos had his entire world-view destroyed by his experience, and etc.
But it still feels like they aren't doing enough with some characters to make them stand out against the background of their homeworld. It makes their being planeswalkers not
mean anything. Partly this is just the issue of them not having the space to develop characters like Tibalt, but narrowing down the focus to a recurring cast
really doesn't help that. But Shadows over Innistrad didn't really have much of a problem, so we'll see how things go in the future.
But really, it comes down to the fact I want to see walkers toss around spells they couldn't learn at home. I want to see them pull out artifacts of foreign provenance, I want them to think differently from their peers because they've seen things the others couldn't imagine.
And you don't need hardly any space to do that.