The Mending changed things. That's probably the understatement of the year in Magic: When the last of the Dominarian time rifts from the Time Spiral Crisis was sealed, on Otaria, mana began to flow normally through Dominaria's lands - and something resonated throughout the Multiverse to rebalance the use of all mana and give new creation a chance, fundamentally altering the nature of the planeswalker spark. To sum up, after Time Spiral, planeswalkers were no longer gods in mortal form (all of them inherently not subject to aging and capable of casting unfathomably strong spells, and most of them gone quite mad from power), but mortal, fallible mages whose sparks gifted them with a much smaller set of gifts, with their new powers restricted mostly to traveling from plane to plane at will.
However, the "oldwalkers" - Urza the supreme Artificer, Tevesh Szat the dark manipulator, Freyalise the nature goddess, and Sandruu the melancholy Minotaur to name a few - still have many fans, due to their outsize personalities and longtime dominance of the lore. And it's hard not to be curious how they'd look (perhaps with slightly lowered in-story power levels) on planeswalker cards. One reader of Mark Rosewater's Tumblr blog had a solution to how such a thing could be seen in a current block.
When subliminallyprintit asked Rosewater, "If Karn can time-travel, is there any chance of a future block that takes place pre-mending?" he answered, as often:
There's always a chance. : )
Rosewater has previously said that time travel stories are hard to do well, especially through the medium of a Magic set - but that it's also one of his favorite tropes. Reactions to the idea were mixed - some commenters wanted the pre-Mending 'walkers to stay dead respectfully, while dilly8021 commented, "This has been my speculation with Karn since the end of new phyrexia. Who better to fight NP than the ones who beat the original Phyrexia?"