In the patch notes for tomorrow's Hearthstone patch, Blizzard has announced that Battlegrounds is getting a huge update to how minions work!
Instead of rotating minions out, there are now pools of minions that will make each game of Battlegrounds feel a bit different! When the game begins, it will tell you which minion tribes are present in that game of Battlegrounds. With 6 currently active tribes in Battlegrounds, only 5 of them will be available each game. When a tribe is removed from the pool for that game, you won't see any cards that have the tribe or certain heroes which were designed around those types of minions.
Since Pirates have just been added, they will always be included in the pool of minions, though Blizzard has stated they will eventually join the shifting pool. Here's what Blizzard has to say about the new feature.
Battlegrounds Shifting Minion Pools
At the start of each game of Battlegrounds, five of the six currently available minion types (Beasts, Demons, Dragons, Mechs, Murlocs, and now Pirates) will form the recruitment pool for that game. The active minion types will be displayed while you’re selecting a Hero—you can also check anytime during the Recruit phase by hovering over Bartender Bob’s deck. To keep things fair, Heroes designed with synergies around a specific minion types will sit out if their corresponding minion type is unavailable.
Pirates will always be available in the minion pool until they join the rest of the shifting minion pool in a future update.