However they approach it, I hope it's not a core mechanic of the game. I like FO4, but I found it tiring having to build so many things for all the settlements so they could asking me for help.
I'm 80% sure it will return. Nothing has been confirmed or denied about the future title though.
If it does return, I kinda hope it's a little less about making a fortress to ward of enemies, and more about just relaxing and making a nice little house. Maybe another location in the game can let you build a fortress as an alternative. Who knows? I just think that after tirelessly adventuring across the continent, the player should be able to sit down and decorate their house to relax and not worry about giants or endless bandits.