The same formulas WotC uses for every block is to

1
a. determine on which plane the next block occurs
b. make mechanics and plot for that block

or

2
a. determine a mechanical theme
b. find or create a plane that matches the theme

An alternative method is to let the story drive the direction of the planes for the next block. This is perhaps the most organic, immersive, and powerful storytelling. This has happened in the past, especially Weatherlight saga and, to some extent, Mirrodin Besieged / New Phyrexia.

Such a block may take place on one or more planes as dictated by the story's needs. For instance, Mercadian Masques block takes place on three different planes simultaneously. However, a common event on all three planes is a rebellion on each plane. Hence all three sets in that block has Rebels, even though each set takes place on different planes.