If you're really into the settlement building aspect, Fallout 4: Vault-Tec Workshop
is the best it gets. For starters, Vault 88 is much larger than any settlement in the core game. There are actual sectors, each based on cardinal directions. So if you've got a ton of materials, a ton of time, and the willpower to do a bunch of micromanagement, you can turn Vault 88 into...well, whatever you want.
Obviously there are all sorts of new things to build and mess around with, and if you're really into playing the cosmetic game and making everything look 100% original, you'll probably find a lot to like here. And hell, even if you're into substance over style, Vault-Tec Workshop introduces a whole new tier of generators, each of which delivers far more output than they ever have. As they relate to settlers, some of the new toys are, ahem... not exactly nice.