I love how some armors look in FNV (Legion centurion armor) with one arm being stripped power armor, why couldn't we make our own armor? by taking some power armor and ripping parts off rendering it useless and taking away from the protection it offers, the part that is a stripped wouldn't be as protective as it was connected to the full suit. And we would be able to do this with most armors with the exception of a few unique armors and fraction armor (mixed fraction armor would cause a lot of bugs).
We should be able to do this with weapons as well strip down two weapons (shotgun and assault rifle) pick one of two types of ammo shotgun shells or rifle rounds, pick a stock shotgun or rifle, the rest of the parts for a gun. Again with the exception of a few unique weapons, special names or special looks.
Also I would love to see a semi customizable house (getting a bit Sim related I know) but a cool hide out is something everyone would like at least most people and what better way to make it cool then let everyone make it their own way? Give a blank house buy your bed and walk to one of a few places beds are able to be placed (highlighted in green) and set the bed get a pop up saying "is this where you would like to place your bed?" Yes or No and allow it to be moved later if you want to move things around if you like.
Making your own ammo, food, drugs is a great thing from FNV I would love to see in the next Fallout, a new addition would be cool to have a bed roll kit from close to the start of the game (for hardcoe players especially since they will need to sleep and fast travel takes days) sleeping in the wild could also be dangerous what with all the animals around and raiders.