Leaked information about Fallout 4 suggests that the game will take place in the Commonwealth, the post-apocalyptic remains of the state of Massachusetts. It's a brand-new location but there may be a few familiar faces there.
There's a chance that a few characters from other Fallout games traveled to the Commonwealth for various reasons. Here are the enemies and friends we expect to return in Fallout 4.
Madison Li"The plan was to build a facility that could purify all the water in the Tidal Basin at once. No radiation, no muck, just clear water."
If you haven't played Fallout 3 in years, you might not remember Madison Li. Nonetheless, she was there for the beginning and end of the game. Li helps deliver the player when he or she is first born. She's also the one who asks them to risk their lives to fix Project Purity.
Project Purity, the pet project of Li and the player's father James, was intended to purify the water of the Capitol Wasteland. Players could make the machine work as intended or modify it to poison all humans exposed to radiation along with ghouls or mutants.
Regardless of the player's decision, Li has left the Capital Wasteland by the time of epilogue DLC Broken Steel. Supposedly she headed north to the Commonwealth.
Li could fit into Fallout 4 in a significant way. Maybe she's intent on launching another Project Purity to clean up the water of the Commonwealth region. Or perhaps she has some other plan to decontaminate the wasteland. Either way, she might seek out the powerful scientific organization known as the Institute to assist her in these plans.
Dr. Zimmer"The Commonwealth itself is nothing but a war-ravaged quagmire of violence and despair. Inside the sealed environment of the Institute, however... But... the Institute's affairs are none of your concern. Your undeveloped mind couldn't even begin to comprehend what we've accomplished."
The Institute is a bastion of scientific research where brilliant minds construct advanced technology. They have a dangerous lack of empathy, though. The organization creates androids but forces these synthetic humans to be their slaves.
Zimmer is the only member of the Institute players have met so far. He comes to Rivet City in Fallout 3 seeking an escaped android. The player can either help him, kill him, or drive him away.
Assuming that he's not dead, Zimmer presumably headed back to the Commonwealth. There are plenty of other runaways for him to track down with his bodyguard Armitage. Fallout 4 players could assist or hinder him on these new hunts.
Harkness"But I'm done with that life. I'm through with being someone's property. I am not malfunctioning! Since when is self determination a malfunction?"
Even if Dr. Zimmer doesn't appear in Fallout 4, perhaps his target from Fallout 3 will. Harkness is an android with an interesting backstory.
At the time of his creation, Harkness was the most advanced android ever built by the Institute. His creators assigned him to the Synth Retention Bureau, an organization that hunts down androids that escape captivity. The Synth Retention Bureau androids are told that they're hunting down androids who fled because of a malfunction, rather than a desire for freedom.
However, Harkness was too smart to be deluded by the Institute forever. He fled to Rivet City in the Capital Wasteland and underwent a memory wipe and surgery to throw off his pursuers. His memory can be restored with the help of the player, though.
After being saved from capture by Dr. Zimmer, Harkness says that he's going to continue working as a security guard for Rivet City. Will that job satisfy him for long, though? Now that he remembers how the Institute treats androids like him, won't he feel the urge to do something about it? The Railroad, the organization dedicated to helping androids escape the Commonwealth, could use someone with his skills.
Arcade Gannon"I don't think you need to worry about the specifics. Basically, expect a lot of firepower and dead bodies."
Fallout: New Vegas was developed by Obsidian Entertainment so Bethesda may not use much of that game's story or characters in crafting Fallout 4. However, the time periods of Fallout 3 and New Vegas are only a few years apart so it would be a shame if there wasn't a crossover between the East Coast and West Coast games. Arcade Gannon is one New Vegas character that could make the jump to Fallout 4.
Arcade was one of the more interesting companions players could recruit in New Vegas. Though he's a doctor by trade and a member of the pacifistic Followers of the Apocalypse, he's not opposed to using violence if need be. Growing up among members of the Enclave, the violent remnants of the pre-war United States government, seems to have made him suspicious of anyone in power.
In one of New Vegas' endings, he leaves the Mojave Wasteland when others learn of his Enclave ties. He "pushed deep into the eastern plains" in order to avoid bounty hunters, the New California Republic and other pursuers. Who knows, maybe he survived the cross-country trek and arrived in the Commonwealth?
Whether or not he'd be willing to join the Fallout 4 main character on his journey will depend on the time period of that game. Arcade might be an old man in the game. Even if he's not recruitable, though, his return would be appreciated.
Three Dog"Because one dog ain't enough, and two is too low, it's me, Three Dog!"
Music is in rare supply after the nuclear apocalypse. Three Dog risks his life to keep the tunes going, though. He set up Galaxy News Radio in the middle of the mutant-infested ruins of Washington, D.C. so he could broadcast pre-war music throughout the Capital Wasteland. He also uses his radio station to keep local residents updated on the player's many exploits.
Three Dog also has a bit of an idealistic streak. He broadcasts the news to counter the radio propaganda of the Enclave. With the Enclave all but wiped out by the end of Fallout 3, will he leave the D.C. area to fight them on another front?
Erik Todd Dellums, the voice actor for Three Dog, hinted that his character may return in a new Fallout game. Perhaps he'll supply the soundtrack for player's journey through the Commonwealth.