One of the most compelling parts of the campaign for me was the theme of family--my favorite moments were always those when I was talking to or adventuring with Heimlock or my siblings. What about an endgame feature that allows you to unlock those characters as companions you can take along on missions/expeditions? Maybe even level them up, equip them with different skills/items?

Doing so would also help support endgame content that could be designed exclusively for full parties, allowing solo/offline players to participate by "partying" with their in-game family.