I think a big problem is the reliance on WOTC's roster of existing characters. It might not be hard to write a new character that has stakes in a particular conflict, but that wouldn't be leveraging the brand, now would it? So you have to fit at least one of the 5ish most-recognized faces into the conflict. And you have to take into account which walkers are slated for exposure in the preceding and proceeding sets. And then there's color balance and mechanics within the set to worry about. And after all that you can worry about how that character fits into the setting.
With all those factors in play, WOTCs main roster should have been written with ease of narrative insertion* front of mind. Dack Fayden and Gideon are to good walkers to have in the roster for that reason. It's easy to stick Dack on a plane- just say he has a shot at some sweet loot. Gideon doesn't like to give up on people, so having him run into some folks in danger is a great way to ground him on a plane- or at least get him to come back for round 2. A dogged detective/explorer archetype, somebody on an assigned mission, somebody who can only planeswalk intermittently, somebody avoiding walking to escape notice of another walker, a figure who can banish walkers elsewhere semipermanently- these would all be good tools to have in the narrative roster. MTG's multiplanar angle pretty much demands at least a few characters who can slot in anywhere. I suspect the Lorwyn 5 and most walkers to follow were built to tick off marketing objectives instead.
Just having planeswalkers able to interfere with one another's walking would be an alright out too. Or put some real weight behind the "planeswalking takes time and energy angle".
*It's a term now. I termed it.
Let's say you want to tick off a color wheel of marketing as well. You could still do it while providing good narrative motivation, and without changing the planeswalking rules (though I *like* the time and energy constraint and wish it was better used)
A
"hero" who won't let innocents come to harm (Gideon) -- Doesn't want to planeswalk while peril or injustice exist for others in their immediate presence
A
researcher that would have to give up on knowledge by exiting the scene (Tamiyo) -- doesn't want to planeswalk until their research project is complete
A
Assassin, who always sees their job through (Think Angel Eyes, from
The Good, the Bad, and The Ugly) -- doesn't want to planeswalk until they've fulfilled their contract and gotten paid.
A
"thrill seeker" with a personally-driven desire (Dack or Tibalt) -- doesn't want to planeswalk until they've achieved their particular form of satisfaction
A
Explorer, who will be found seeking some sort of natural wonder -- Doesn't want to planeswalk until they've experienced and bonded with the object of their journey.
You can apply whatever tick boxes you feel compelled to after that, but those characters will still be united by possessing personal goals