The problem with the Kamigawa flavor is that it works well in individual cards and horribly collectively. The Kami collectively are very hard to distinguish and the Arcane spells all have weird abstract concepts that are totally different from how the same effects are typically flavored -- but then even with all this there also isn't the level of diversity in the setting because there's no gold cards and almost all tightly-constrained tribes and affiliations in each color.
It's like perfectly positioned for people to discover the cool parts of by accident later on, but come off really poorly when it's 100% of what's on offer for a whole year.
Kamigawa works better as a Japanesque fantasy album project than full-scale worldbuilding or game design. An actually well-realized Japanese setting will not be nearly as abstruse and unrelatable, simply because it will be deeper rooted in how real people live, think and behave, rather than dreamt up by Westerners who read a few books on samurai and yokai.
In the world of Overwatch, there is incredible diversity. ... They all play different. They all feel different.
Magic is handicapped here on so many levels:
... You have to constantly print new cards, which mean you have to constantly mechanically reinvent your face characters. In other games, maybe your favorite character gets better or worse as the meta evolves, but it is the exception rather than the norm for your favorite character to play differently over time. What happens if I love Nissa as a character, but what her latest Planeswalker card does is mechanically uninteresting to me?
The theory I've been rolling around in my head this year is for each "Magic year" (i.e. fall to fall) come up with a "basic" set of like 30-40 cards, give them a custom frame treatment, and insert the same set of them at X frequency in all four sets from that year -- they already do a thing like this with Masters, so no reason they can't also use it for another purpose.
So if we are to synthesize these ideas without turning Magic into a living card game:
These annual Essential sets, inserted into the main sets rather than sold separately, can be flavored as the skill set of our protagonists (the Jacetice League) and their rivals (if their involvement is already known at the outset), leaving the main set to entirely focus on introducing new things: the current plane, a few storyline cards, new planswalkers, and the villain's nefarious new schemes you can never guess.
They will be ALL about highlighting the backgrounds and personalities of planeswalkers. Their frames should clearly distinguish them from the current plane, similar to how the walker cards stand out as outsiders when they were first introduced in Lorwyn. I don't know if they should include the walker cards themselves, but they will definitely showcase the walkers' signature spells and favorite summoned creatures. Their artworks can be set in their home planes, as long as they don't conflict with the current block's mood. E.g. During a block set in Kaladesh, Liliana-themed cards can have artworks that reference Innistrad, as long as the gothic horror flavor is not too explicit. The feeling should not be too different from mixing Core and Expansion cards together in the past.
Meanwhile, the story can be kept on a soap opera style treadmill (the "soap wheel"), with multiple parties constantly forming and regrouping, always running into new trouble.
Ideally this should be coupled with a shift away from constantly churning out new walker cards as the selling point of each set: new walker cards are only designed when needed rather than mandatory; well-designed cards become emblematic of their respective characters and are frequently reprinted and used as anchor points in balancing, while unsuccessful cards are winnowed out.
Example:
Block 2019-B: Gideon, Kiora, Liliana, Chandra and Nissa visit Atlazan. They meet a local White planeswalker, and discover Tibalt is causing havoc.
Magic Essentials 2019:
White - Gideon's Theros kit;
Blue - Kiora's Zendikar and Theros kits;
Black - Liliana's Dominaria kit;
Red - Chandra's Ragatha kit;
Green - Nissa's Zendikar kit.
After the block, Standard rotation happens.
Block 2020-A: The Atlazan planeswalker joins the party; Gideon and Chandra leave to investigate another lead; the party chases Tibalt to a more populous world, and meets up with Jace; Nissa switches to using Kaladesh-inspired spells, which are better adopted to the urban environment.
Magic Essentials 2020:
White - new character's spells from Atlazan;
Blue - Jace's Ravnica kit;
Black - Liliana's Innistrad kit;
Red - Tibalt's Innistrad kit;
Green - Nissa's Kaladesh kit.