I'd like a couple more personality types for settlers to breath a little more life into my settlements. The eyebot pod is one of my favorite things, so more utility functions that add practical benefits to base building would be cool too. It would be interesting to have another new faction added as a rival to the Minutemen/Player, and have that faction start moving into settlements the player isn't using and setting up shop. This would give you something else to do after the main story, work against the opposing settlement faction to try to unify the wasteland. It would also be cool to be able to negotiate with and broker deals and trade agreements with already existing settlements that you aren't allied with. That way you could have a defense agreement with Bunker Hill, a trade agreement with Diamond City, and a protection agreement with the Brotherhood of Steel, even if you aren't necessarily allied with them.
For crafting, it would be a cool addition to add some unique player-craftable weapons, armor, or items. Things that won't be found in the wasteland and that other characters won't be using (like how the railway rifle, junk jet, and dart gun were) but that the player can find schematics for and build. With Automatron I didn't realize I wanted to build robots and mix and match parts until I tried it, and then the pokemon syndrome kicked in and I made farmerbots, and guardbots, and cargobots, ect. Being able to make player unique pieces of armor, weapons, and maybe even one-shot items could be the same way.
Also, what about introducing another vehicle type? Right now we have Power Armor which is like a wearable vehicle, and there are Vertibirds which are a cool vehicle available to BoS players. Could we maybe see the addition of a garage workstation and a reintroduction of the Highwayman or a Corvega vehicle? It would be pretty cool to use a system that is sort of a hybrid between power armor modding and the automatron robot building to make our own custom wasteland cars/trucks. This would be even MORE useful in survival mode where the amount of guns/ammo/supplies you can carry is more limited and the amount of time it takes you to travel is greatly increased. It'd be great to load up the trunk with everything you need for a three-day journey across the Commonwealth and have a little mobile base you can drive around with for the trip.
I'd also like the ability to get a little bit deeper into the nuts & bolts of what my settlers are doing and make some settings to change their default behaviors. Something i've noticed is that settlers on guard will often patrol around, sometimes get stuck, and won't always hit the alarms I set up at the right times. Being able to select an assigned duty station, shift assignment, piece of furniture, or manually set settler pathing would be really cool. It would also be great to be able to assign a settler as a guard without building a guard post. Something I do in my settlements now is have the first room in my living area be a sort of security checkpoint, so that ne'er do wells can't run in and kill people while they're sleeping. Being able to assign a guard to the security desk inside the entrance for an 8 hour shift and have a rotation so it's always guarded would be great. Being able to also assign switches or terminals to a duty station would be good too, this way to get let in to the living area you would talk to the guard behind the fencing and say "Hit that switch" or "open this door" and they would let you in.
That's just a couple of ideas that would be cool