Parmandur
Book-Friend, he/him
Thanks, this is very helpful. I would guess, then, that Zendikar, Innistrad, and Dominaria are as close to definite--and possibly in that order. Doesn't Zendikar lend itself well to high level play? Hmm...
The Levels 4-5 settings are all possibilities, but very unlikely to be in the works.
Where does Ixalan fit?
Well, Magic is putting out three Setting focused Sets a year, so the chances that they will visit any given 4-5 Setting eventually are quite good, and some of those (like Amonkhet, Eldraine, or Tarkir) would make excellent D&D Settings, so I wouldn't discount them in the long term. But yeah, those three are the big, likely culprits.
Per your musing about ranking the old D&D Settings, these are the ranking criteria that Rosewater has laid out on an old article (the precise rankings are somewhat outdated, he makes adjustments over time):
The Storm Scale was so popular on my blog that it has spawned numerous other scales. This article and next week's column are about one of these—the Rabiah Scale. The Rabiah Scale is about how likely we are to return to a plane in a Standard-legal set. It's named after Rabiah, the setting of Arabian Nights, because Rabiah is a 10 on the scale (aka we're very unlikely to return there). In today's column, I'm going to explain the various factors that go into predicting whether we'll return to a world and then rate each one on the scale, with 1 being very likely and 10 being very unlikely.
Here are the criteria I will be using:
Popularity
This criterion is very similar to the Storm Scale. Basically, did the audience like it? The more all of you like a particular world, the greater the chance of its return. I'm using data wherever I have access to it, but some of the earlier worlds, I'm using more anecdotal evidence as we didn't do market research in the early days.
Very Popular – This means that through our market research, this world falls in the top 25% of worlds of all time. Note that all these categories are comparing the current worlds against all the worlds we've done (well, since we started doing market research many years ago), so getting in this top section is difficult.
Popular – This means that through our marketing research, this world falls above the average, but not in the top 25%.
Liked – This means that through our research, this world falls below average but not in the bottom 25%. I should note that we aim for our average to be well liked, so being below average doesn't mean the majority of players don't like it, just that there are other worlds they like more. Being in this category doesn't automatically keep you from having a chance to return.
Unpopular – This means that our research puts this world in the bottom 25%. Falling in this last section does decrease the chance of a return.
Mechanical Identity
This category examines whether the world has mechanical associations. Is there something for us to mechanically design around? Is there a component that the audience associates with this world that would create excitement for its return?
Strong – This means the world is strongly associated with one or more mechanical components. This makes it easier to design, more likely to excite players, and increases its chances of a return.
Average – This means the world has some mechanical associations, but they are not as strong as they could be.
Weak – This world has little to no mechanical associations. This decreases its chances of a return.
Creative Identity
This is the companion to the mechanical identity category. Instead of looking at mechanics though, this category looks at the creative elements—the cosmology, the look and feel, the inhabitants, the locations. Do all of them have a strong cohesive feel that makes the world feel like a single unified world? The more the world has a creative cohesion, the easier it is to return to it and the more excited the players will get upon learning of a return.
Strong – This world has a very clear identity. For example, if you saw a random card, you'd know in a heartbeat that it came from this world.
Average – This world has an identity, but one not quite as crisp and delineated.
Weak – This world's identity is not as cohesive as we like worlds to be.
Room for Expansion
This category examines what space there is to create new things in this world. Sets, even returns, have to introduce new material. Is this world set up to allow new exploration? A big part of this category is how much the earlier visit(s) did to hint at other aspects of the world. The more room for expansion, the easier it will be to return there.
Significant Room for Expansion – This means the world has many new things to explore in a return.
Some Room for Expansion – This means the world has some new things to explore in a return.
Minimal Room for Expansion – This means the world would struggle to find new things to explore.
Story Continuation
This category talks about what story threads were left last time we were there. In other words, does returning to this world allow us to finish telling a story we started when we were there last? Note that this is weighted by how much the audience cares about a certain storyline.
Major Plot Line – This world set up a story or stories that players have been asking about.
Minor Plot Line – This world set up a story or stories that players have been asking about, but less.
Minimal/No Plot Line – This world set up a story or stories that players have not been asking about or didn't set up any stories.

The Rabiah Scale, Part 1 | MAGIC: THE GATHERING
Mark uses the Rabiah Scale to examine how likely he believes we are to return to a particular plane in a Standard-legal set.