Look, there are plenty of reasons to dislike Fallout 4's settlement building system. Just trying to put something together that wasn't floating ten inches off the ground took hours of planning and a hell of a lot of trial and error, so it's no surprise that most players just ignored the whole thing as best they could.
But while it took way too much precedent in the game's main quests, at its core, settlement building could be a great addition to Fallout as a series.
Being able to establish and cultivate a safe haven from scratch Would provide a huge alternative way to play a game like Fallout, and if it became a more persistent mechanic - think attacks from increasingly stronger threats trying to steal your resources as you grow in power - then a new and improved settlement system would give players hours and hours of extra content long after the main missions have dried up.
Of course, not everyone is going to enjoy running back to their base every ten minutes for yet another defensive firefight, but keeping this element optional could provide a huge draw for those interested in a more emergent and spontaneous style of gameplay.