Looking at forge world's site has got me thinking, who's the most likely for their next primarch model?
They are starting to release Salamanders models, but also have put out the fortress tiles and legion heavy artillery. They also have a lot of the Death Guard infantry specialists out, as well as the Typhon character.
Other than Angron, who came out before almost all of his legion specific models, almost all of the others followed unit releases.
That means Mortarion, Vulkan, or Perturabo is likely next.