Horizons gets you access to everything in the Horizons "Season", which consists of the Planetary Landings update (2.0), the Engineers update (2.1), the Guardians update (2.2, currently in beta), the Commanders update (2.3, forthcoming, includes an avatar editor and multicrew gameplay), and an unnamed 4th update. It should be noted that 2.2 is massively behind schedule at this point*, and there's no date yet for 2.3 and 2.4.
Presumably, the 3.x series will again be a season pass purchase.
EDIT: * Massively behind schedule in Elite terms, not SC terms; 2.2 was originally announced for the Summer, and is coming out sometime this month