I am on a mission to think up a continuation of Lost Mine of Phandelver, because it's a very fun campaign and it's easy to set up and run. My main goal is to keep the expansion in the same storyline, with the same feel, basically not have it diverge as heavily as some of the others i've seen in terms of vibe. The solution i came up with, with very little modification to the campaign as written, is to make the doppelgangers have a bigger plan then they let on.

In the campaign, there are two doppelgangers who are working for the Black Spider, who are kind of unused considering all their potential and all the unique personalities in Phandalin whose visages they could usurp. To start from the beginning, my idea is that 500 years ago, it wasn't orcs who attacked the mine, it was doppelgangers who needed a scapegoat, so they threw orcs under the bus. They were tired of all of the immediate suspicion that always befalls them, and wanted to weaken all the humans and others who keep killing their kind. They stole an important element from the Forge of Spells, a power source that allows its magic to be strong enough  to quickly and permanently enchant items. After the raid, they disguised themselves as popular figures in order to properly spread the lie that orcs conducted the attack, and that's what went down in history.

Half a century later, that power source still under constant guard, and doppelgangers' standing not getting any better in the world (except of course those who made it big impersonating already famous people), they hear that someone has rediscovered the mine. At this point, so many generations later, the location has been lost to them too, and the part still dedicated to this cause has become much smaller and more select. So they recruit a guy, some weakling desperate for a big shot, calls himself the Black Spider, to go figure out what's up, find the one who discovered the location, end him, and tell him yeah he can totally use the mine as his own once he finds it (they are, of course, lying). They send two of their own with him.

Now, the campaign plays (almost) as normal. The doppelganger gang doesn't seem too worried, so they don't get involved. The one change to the book, is that you pick one or more people in town, have them killed and replaced with the doppelgangers. My personal choices are Townmaster Harbin Wester, and the barkeep at the Stonehill Inn. These are people the party are likely to interact with at some point or another, and people who are likely to hear of the party's great exploits (aka info to report back to their leader). Have the characters' original corpses be hidden poorly somewhere for the players to find them, and let the suspicion begin. Now, along with being constantly watched by the Black Spider, which they should know about by now, they also suspect anyone in town of not being who they seem, and will soon connect these imposters back to the Black Spider.

At the culmination of the written campaign, give the players the opportunity to find that dungeon map, which is now a map of the dungeon in which the doppelgangers guard the power source for the Mine, dropped by a raider who died 500 years ago. They find and save the mine as normal, possibly killing both doppelgangers or not. Whichever Rockseeker brothers are still alive try to reopen the mine, and offer the party a job protecting it, and from whoever you deem fit, they also receive the deed to Tresandar Manor. Now it's downtime time. The Rockseekers ask the party if they can figure out where the power source of the mine is, as it's missing (a reflavor of the magic of the Forge of Spells being so weakened). The doppelgangers are collecting their forces, they're gonna need to do another raid, their liason failed. So we pull a Dragon Heist and have them fireball/dragon attack/otheriwse attack the party's new home in an attempt to kill them, not realizing just how strong they are. The party fights, figures out it was doppelgangers who did it, check in on the Mine and the Rockseekers, maybe there are remnants of a battle, maybe one's still going on, and from there follows the investigation into what these doppelgangers have to do with this mine, and where the power source is. They probably have the map of the dungeon right now, and know that doppelgangers are abound, so the next step is to make connections discover their motivations, all the while the doppelgangers back at their base are starting to build up power to fight back, because now heroes are coming after them and are searching for their location, the exact thing they feared. Now it's a goose chase for the heroes to find the doppelgangers' lair, and the doppelgangers to try and kill them or lead them off the trail.

Along the way, the party will learn more about the true backstory of the mine and the doppelgangers' reasons for destroying it, and may decide to make a truce or just kill them all. yknow. up to them.

This is more of a restructuring of the campaign for a set-up for a bigger story, rather than a detailed module add-on, but i think it's fun and i'd like to run it like this in the future. i guess any feedback? additional mission ideas? literally whatever lol