Pathfinder playtests give you the opportunity to participate in finalizing rules and new materials, including new classes, spells, ancestries, feats, and more!
Paizo generally releases system splatbooks in a tick-tock/major-minor cadence. Around GenCon each year, they release their major splatbook, which usually has new classes in it, as well as significant archetypes, items, and subsystems built around the theme of the book. Then there's a minor splatbook released the following spring, which doesn't contain new classes, but comes with more thematic archetypes, items, and subsystem expansions.
Last year, we got Rage of Elements at GenCon, an elemental/Primal major release which had the Kineticist in it. This May we're getting Howl of the Wild, a minor Primal/nature book that contains animal hybrid ancestries (minotaur, centaur, merfolk, etc.) as well as awakened animal and insectile ancestries, plus werecreature archetypes.
This fall, a little later than usual, we're getting War of the Immortals, which contains the Animist and Exemplar classes, which were playtested last fall.
Next summer, we'll be getting Battlecry, which will contain the final versions of the Commander and the Guardian.
Player Core 2 is set for an August release, coinciding with GenCon this year, and will contain updated versions of the Alchemist, Barbarian, Champion, Investigator, monk, Oracle, Sorcerer, and Swashbuckler classes.