I mean when creating the action, it could be possible to use some of the event's properties as the input.
Example: Increment player statistic: Change by (whole number)
com.playfab.player_statistic_changed.StatisticValue or only StatisticValue
Detailed Example: I want to have several level packs in my game and for each of them, I have a statistic that tells me, how many levels of the pack the player has played. I also want to have a statistic that will show how many levels the player played in total (a sum of all the statistics for each level pack). I know this is achievable through Cloud Script, but it seems to me like a really good and powerful enhancement to PlayStream. So I would create an action on player_statistic_changed with conditions that will match my level packs statistics and the action will be Increment player statistic TotalLevelsPlayed by value
StatisticValue - StatisticPreviousValue