The Planeswalker is possibly the most important aspect of Magic: the Gathering lore and I feel is the one thing that needs to be handled first in this project. Ever since reading Plane Shift: Zendikar I began to work on just how to handle the idea of Planeswalkers in D&D. Exactly how these beings would present themselves in the D&D universe and how they could be integrated into a campaign and group would need to be one of the first issues tackled. Sadly, there is no easy answer to beings as powerful as Planeswalkers in D&D.
Planeswalkers as Magic sees them
The history of the M:tG Planeswalker begins with the idea of Planeswalkers being immortal, godlike beings that represented the players of the game. Their forms were the expression of their wills and they were some of, if not the, most powerful beings in the multiverse.
This was until the Time Spiral event where Planeswalkers were toned down and were now just powerful wizards that were now mortal. Yes, they could still move through realities and cast powerful spells but they were now mortal, aged and were limited by many of the same things the 'lesser' beings are.
In both versions the necessary means to become a Planeswalker is the Planeswalker Spark that only a few beings ever carry and even less see ignite. So becoming a Planeswalker could be a one in a trillion chance. While this works for Magic in D&D this becomes a problem as it becomes virtually impossible for any character to become a Planeswalker.
However using the above and making some adaptations we can get something going here. While it may not be pure Magic lore it will work for our purposes.
But first. . .
Planar Travel in D&D
Compared to the above, planar travel in D&D is much easier. When you have worlds, and a universe, with planar gates, portals and doors as well as magic items and well known spells it becomes much easier to hop from reality to reality.
No spark is needed to traverse the planes and all you need is some knowledge of how to use certain events and items to your favor. Then once you find your way into these realities all you really need to do is make it into Sigil and you are at the center of everything.
From there it is a matter of just collecting new knowledge, spells and magic to transcend to a new level of being and become even more powerful that the new breed of Planeswalker has become.
Fitting them together
While thinking on how to fit this together I came upon a couple of ideas that would fit the old Planeswalkers, new Planeswalkers and the easier travelling in D&D together in some form. While this breaks a lot of canon and lore in both universes it does work and creates some interesting adventure ideas in the process.
Let's begin with the old Planeswalkers and the Time Spiral.
With the temporal breakdown on Dominaria and the rest of the multiverse a few of the old Planeswalkers saw what was coming and the destruction of the original spark. Not wanting to see the end of their power they began looking for a way to escape what would befall them.
It was during the planar chaos of this time a few of these Planeswalkers found another universe that was seemingly unaffected by the trouble in and around Dominaria. Using the rifts in their multiverse these Planeswalkers escaped Dominaria and came to what we know as the D&D universe/multiverse.
However, they made a miscalculation and arrived in the pasts of many worlds. On one such world, the Planeswalkers known as Bane, Bhaal and Myrkul established themselves as gods. On another the Planeswalker Vecna built an empire and unknown to all, the Eldrazi Tharzdun appeared in the primordial universe but was imprisoned by a mixture of gods and Planeswalkers.
Unknown to the citizens of this multiverse, many of their gods, demon lords and dukes of hell were at one time original Planeswalkers. While they may have lost some of their power in the move they did not suffer the consequences of The Mending as their Dominarian kin.
With the old Planeswalkers established as gods or demigods this leaves the question of the new Planeswalkers and how the spark would manifest in the D&D universe. Given the D&D rules and the rarity of the spark I would say that it only would manifest under a certain set of conditions.
Perhaps maybe with certain spells, items or maybe even as an epic boon of sorts. Another option would be that Planeswalkers manifest differently in the D&D universe than in the Magic universe. Maybe the combination of certain high level spells and magic items would be enough. And with the various portals on many of the worlds you do not need to have a spark to be a Planeswalker.
Of course, this is all speculation and thought exercises at this point. But you could rework high level NPCs like Elminster and Rary to be the new breed of Planeswalker in the D&D universe. The final point is that Planeswalkers in D&D would be different than their Magic counterparts and this should be so.
Next would be how the D&D multiverse would look in this exercise (Here's a hint, think 3.5 D&D), how to build a Planeswalker using D&D rules and how famous Planeswalkers would look in the D&D universe.
After that foundation it is on to a core world and how to bring creatures, spells, artifacts and items into the game. Yes, it is a long process but it will be worth it.