Popularity: Khans version (Popular), Dragons version (Liked)
Tarkir is in a weird place. We visited a plane, then radically changed it, and the audience preferred the world before the change. Luckily, we seeded a few things into Tarkir in case this happened.
Mechanical Identity: Strong
Tarkir and its wedge color identity (a color and its two enemies) are strongly linked, and I doubt we'd return to Tarkir without a wedge presence in the set. (Although, I should note that three-color has some play design issues to work through.) Tarkir is also linked to Dragons, which would also be a component on any return.
Creative Identity: Strong
The Creative team spent a lot of time and energy filling the world of Tarkir with amazing creative components. It's probably the most fleshed out world that we've only visited once.
Room for Expansion: Significant Room for Expansion
Tarkir has an extensive amount of worldbuilding, with each clan having its own portion of the world in which to shine. The original block had elements from two timelines and two different points in time. I am confident that a return would have plenty of space to work with, both mechanically and creatively.
Story Continuation: Minor Plot Line
Because of the time travel component of the story, the Creative team was careful not to tie Tarkir too closely to the current storyline, but the world has a lot of threads a story could pick up on, including Sarkhan and Narset, who are from Tarkir, and Ugin who made it his home plane for many years.
Rabiah Scale Rating: 4
The world has strong mechanical and creative ties, is popular, and ties into the modern story in numerous ways. The biggest issue is how to capture some of the old timeline mojo in the current timeline, but a bunch of work was seeded to help us do that.