Each of the 10 Color Combinations was planned to primarily represent a different Plane theme, but there's bleed-over into other colors in the final arrangement.
B/U: Innistrad
B/R: Regatha
B/G: Lorwyn
B/W: Dominaria (GET IT, BLACK AND WHITE, OLD TIMEY)
U/R: Kaladesh (Steampunk India)
U/G: Ravnica
U/W: Vryn (Scotland with Stargates)
R/G: Zendikar
R/W: Theros
G/W: Bant
Beyond planar-dabbling, they're also making a lot of cards that are "reworks" of classic cards, like Days Undoing (Timetwister) and Evolutionary Leap (Survival of the Fittest), as well as cards tied to individual planeswalker background stories.