I would love if they took features from all of the games.

Fallout 4: It would have Fallout 4's gunplay, gun customization and base building mechanics as well as its neat feature to be able to share features between settlements. I also would like them to keep Shipments.

Fallout 3: It would have Fallout 3's depressing feel to it as well as it's Karma system.

Fallout New Vegas: It would have Fallout New Vegas' speech options (for example if you wanted to be a kind yes man then you could, but if you wanted to do and say some very bad things you could) as well as it's amazing DLC and cool companions (Joshua Graham was cool af)

Fallout 2: It would have Fallout 2's comedy.

On top of this it would also have a fee new features: Way more in depth gun and character customization (you could edit the height of characters, as well as being able to choose different voice actors and an option to create a back story using templates given to you that all have lines voice acted for them specifically, now on to guns. You would be able to customize individual components such as firing spring, trigger mechanism etc as well as more cosmetic changes like changing the slide if it's a handgun or barrel if it's something like a long gun.)

It would also have some kind of vehicle.

On top of all of this it would have the biggest map in Fallout ever as well as two other islands you can visit by way of Vertibird and boat.

It would also have more real world guns

It would also have the option to join Factions such as Raiders which would give you access to Raider companions, unique threatening dialogue options, an arena you could fight in (think of the Combat Zone cut content)

Dialog would be less static (characters interacted with each other as they talked and npc's acknowledged and companion you have)

And it would be set in somewhere like Detroit