Fallout 4 has perhaps the most dramatic introduction of any Fallout game. The Sole Survivor, whether that be the mother or father of Shaun, actually predates the war, surviving the nuclear apocalypse by hauling ass to an open vault and then cryogenically freezing themselves to awaken hundreds of years later.
As cool as that intro sequence is, it’s not exactly realistic. The player character is actually above ground when a nuclear explosion goes off and only survives because the vault’s elevator dips below ground level just as the shockwave arrives. The reality of being anywhere near a nuclear explosion when it goes off is you’d be vaporized almost instantly with no waiting required.
So, to make Fallout 4's intro far more realistic, modder Unreal Septim decided to make the More Realistic Intro mod (with thanks to PC Gamer). "This mod makes one simple but significant tweak to the game's intro sequence in order to make it more realistic," they write in the mod description. "New game required."
What does the mod change? Instead of running to the vault and arguing with the guard about whether you're authorized to survive Armageddon, you die. Right after the news report of New York getting atomized and just as the player steps outside their home to start running to the vault, a nuclear bomb goes off, and then the screen turns to the same "Please Stand By" message that the news report did.
"This has two major gameplay effects," Unreal Septim adds. "One, it makes the intro sequence much more believable. Two, it effectively prevents your character from progressing beyond the intro sequence, making the game essentially unplayable."
Septim suggests sending this mod to a friend without telling them what it does for a fun time, although livestreaming their gameplay would be required for best results.
For a mod with a little more playtime, might I suggest "A Taste of Blues"? It's a mini-expansion based on Fallout: New Vegas's wildly popular Old World Blues DLC. It adds new quests, new dungeons, 14 new weapons, new clothes, and over 40 rewards, recipes, and items. You can also survive, which is more than what I can say for the previous mod.