Several people expressed interest in "what happened to [character]?" and it can be fun to toss around theories.

Here's my ideas (took about an hour to whip this up):

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• The Count and Haydee: Spent some time traveling around the world. Due to the contacts he's made from 1829-1838, they are warmly received wherever they land. Eventually they settle down in Janina (which was not incorporated into independent Greece) and the populace demands that Haydee takes her spot as Queen/Empress/Governess. The Count provides bribes to the Ottomans to stay away.
• Danglars: Chastened by his experience with the bandits, he retires to a little town in Italy. He cannot return to France because of embezzlement, financial crimes and angry creditors. 50,000 francs is enough to buy a small house with ONE servant and he lives his life quietly, ensuring that his larder is always filled because he cannot stand to see an empty cupboard.
• Mercedes: After a period of weeping and praying, she ventures into town. One of the gentlemen who bought out Morrel & Son is a widower with 2 children (wife died of consumption). He notices Mercedes, her posh accent shows that she's a lady and not a peasant, and they become friends. She learns to live again, the gray in her hair grows out, she starts eating better, and with the money Albert sends, she dresses better and regains her glow. The shipowner proposes and they marry, and she becomes the well-loved stepmother of the children and lives in an upper-middle-class household. Now filled with love and new purpose, she can live for the present and stop weeping for the past.
• Villefort: Spends some time in an upper-class mental asylum. Valentine and her husband Max, accompanied by Noirtier, visit him. This helps him heal, and he regains some of his sanity. He is released from the asylum and returns home with two full-time caretakers. Val and Max visit every year and bring their growing family, allowing Villefort to meet his grandchildren. Any thoughts of revenge against Dantes evaporate when Val explains how the Count saved her and Max, and that's why their kids are now sitting on Villefort's lap.
• Noirtier: Once Max and Val buy a home in a Paris suburb, he moves in with them. The baby Morrels adore their great-grandpa and rapidly learn his language. One of the kids invents a method to radically speed up communication 4x by using 5 groups of 5 or 6 letters each. So instead reading off the entire alphabet, waiting for a blink, they can go: Group 1? 2? 3? 4? 5? (blink) OK, within group 5... U? V? W?(blink). OK!
• Jacopo: His employment with the Count is over, so he uses his little ship to become a legit shipping merchant dealing in high-end luxury goods.
• Julie and Emmanuel: Living a comfortable middle-class existence. They are surprised receive notification of an "inheritance" from a previously-unknown "Herbault relative".
• Maximilian and Valentine: They marry in Leghorn, with the full blessings of Noirtier. Max is still obligated to serve his term with the Spahis, so he buys a home in the suburbs of Paris for Val and Noirtier to live until he can come home permanently. Once he retires from the army, they have a brood of baby Morrels.
• Franz D'Epinay: Returns to Italy, and marries a suitable girl.
• Ali and Bertuccio: Accompany the Count wherever he goes.
• Albert de Morcerf: Becomes Private Herrera in the Spahis. Captain Max takes Albert under his wing and gives him specialized training. Albert is very adept and rises in the enlisted ranks. Max also introduces Albert to officer corps in the Spahis, so when Max's contract is up, it is unanimous that Sgt. Herrera should be promoted to an officer.
• Luigi Vampa, Peppino and Beppo: Since they are allowed to KEEP Danglars' 5 million francs, it's enough to leave behind the dangerous occupation of banditti (which could eventually get them arrested and executed). Vampa takes a new name, marries Teresa and lives well. Peppino and Beppo take their share and enjoy life. Rumor has it that there's an attraction, as Beppo is too convincing with cross-dressing. The remainder of the gang is given 100,000 francs each and the fearsome gang disbands.
• Benedetto: Thought he was going to be sent to the galleys of Toulon and maybe he could escape again. But an entourage of outraged Corsicans (Bertuccio sent them a letter) point the finger at Benedetto and they demand justice for beloved Assunta's death. This, added together with murdering Caderousse, gets Benedetto a date with Madame Guillotine.
• Madame Danglars: Follows Debray's advice, but hides her jewels and has her house watched by a caretaker. Goes traveling all over France and Italy and picks up a new boy toy.
• Lucien Debray: Loaded with money from stock speculations. Now that his affair with Mrs. Danglars is over, he finds a new mistress.
• Eugenie and Louise: Become a HUGE hit in Rome's music theater scene. Tickets are always selling out. Eugenie is showered with flowers and gifts and proposals, and all of these male admirers are left to wonder why Madame Eugenie never seems to accept any offers to court her.