Either Branch and Claw or Jagged Earth next. Both add an essential mechanic (events) with Jagged Earth having more content.
After you get either, the order of the rest does not matter too much, though if your group would prefer more simpler/straightforward spirits, the standalone game Horizons of Spirit island contains good, simple to play (but fun!) spirits that can be used in the main game (and also provides a slightly simpler version of the game to introduce new players with).
Personally I'd suggest Jagged Earth > Branch and Claw > Feather and Flame > Nature Incarnate, and Horizons if you want simpler spirits or run out of other content to buy and need more.
Either Branch and Claw or Jagged Earth next. Both add an essential mechanic (events) with Jagged Earth having more content.
After you get either, the order of the rest does not matter too much, though if your group would prefer more simpler/straightforward spirits, the standalone game Horizons of Spirit island contains good, simple to play (but fun!) spirits that can be used in the main game (and also provides a slightly simpler version of the game to introduce new players with).
Personally I'd suggest Jagged Earth > Branch and Claw > Feather and Flame > Nature Incarnate, and Horizons if you want simpler spirits or run out of other content to buy and need more.