My guess is Bounty Hunter ships will be required. There's a nice parallel there with Thrawn/Chimaera.
Thrawn - Legendary Event requiring Phoenix characters
Thrawn's ship - Legendary Event requiring Phoenix Ships (+ filler rebels)
Chewbacca - Legendary Event requiring Bounty Hunter characters
Chewbacca's ship - Legendary Event requiring BH ships
If they make it "Scoundrel ships" to have more, that makes sense from a game faction standpoint, but less sense from a Star Wars theme standpoint. For Chimaera, it made sense that other rebel ships would support Phoenix squadron. For the Falcon, it makes less sense that an older version of the Falcon would be helping Bounty Hunters track down itself.
I guess one of two scenarios:
1) Legendary, unlock at 5 stars - requires all 4 Bounty Hunters.
2) Legendary, unlock at 5 stars - requires minimum of 3 BH ships, but the difficulty is tuned hard enough that it will be VERY difficult to get 7* with only 3 ships.