Not only does Fallout 4's Project Valkyrie mod add 20 new quests and a handful of dungeons, bunkers, and player bases to the Commonwealth, it lets you alter the base game's ending.
"Want to imprison Preston and execute Desdemona while brokering peace between the Institute and the Brotherhood?" asks the mod's co-creator Thuggysmurf. "Kill everyone? Let the four major factions co-exist in harmony? Or something in between? Now you can."
Thuggysmurf then links to this page which outlines a number of alternate ending scenarios. I'll let you leaf through those yourself, but know that Project Valkyrie lets you become the Director of the Institute and free the synths; challenge Elder Maxson for control of the Brotherhood of Steel; and determine the fate of characters who otherwise live or die in vanilla Fallout 4.
Thuggysmurf says the mod works regardless of which point in the game you're at, or which factions you've aligned with.
"You can play on a new save or any existing save," they add. "However, if you want to play 100 percent of the content, the Prydwen and Institute will need to be intact. If both the Prydwen and Institute are ashes, two of the main quests will be an hour shorter."
Projet Valkyrie also boasts four fully-voiced companions. Have a gander at those in motion:
More information on all of that, including installation instructions, lives on Project Valkyrie's Nexus Mods page.