[X] Don't have the possibility of other 'walkers turning up.
--[X] But have stories / rumours / myths of them be something that can be encountered
Reasoning:
1) Doylist: This quest has tons of stuff to deal with and cover already. There are beings and organizations that will be able to swat Jade like a fly for a long time yet, and, besides that, tons of non-combat stuff to cover. There's no need to overcomplicate this with other planeswalkers.
2) Watsonian: The multiverse is a vast place. Each plane is a huge universe by itself. The probability of two planeswalkers meeting in such a setting is virtually nil. Jade might stumble upon the tales of ancient (demi)gods who shaped worlds, struck down titans and created wonders. She might walk the paths once explored by Urza and Yawgmoth themselves. But will she ever encounter any of her own kind? Unlikely until she can observe the whole planes at once, and travel to any point in them almost instantly. Will she get noticed? Until her passage shatters galaxies and leaves planes trembling in her wake, almost certainly no.
Thus, I think, from the perspective of KISS and from statistical viewpoint, it's easier and simpler to keep Jade the only planeswalker currently active in the local corner of the multiverse.
--[X] But have stories / rumours / myths of them be something that can be encountered
Reasoning:
1) Doylist: This quest has tons of stuff to deal with and cover already. There are beings and organizations that will be able to swat Jade like a fly for a long time yet, and, besides that, tons of non-combat stuff to cover. There's no need to overcomplicate this with other planeswalkers.
2) Watsonian: The multiverse is a vast place. Each plane is a huge universe by itself. The probability of two planeswalkers meeting in such a setting is virtually nil. Jade might stumble upon the tales of ancient (demi)gods who shaped worlds, struck down titans and created wonders. She might walk the paths once explored by Urza and Yawgmoth themselves. But will she ever encounter any of her own kind? Unlikely until she can observe the whole planes at once, and travel to any point in them almost instantly. Will she get noticed? Until her passage shatters galaxies and leaves planes trembling in her wake, almost certainly no.
Thus, I think, from the perspective of KISS and from statistical viewpoint, it's easier and simpler to keep Jade the only planeswalker currently active in the local corner of the multiverse.