Louis XVI, King of France and Navarre (b.1754:d.1793) m. Maria Antonia of Austria (b.1755:d.1793)
1) Marie Thérese, Fille de France (b.1778:d.1855) m. João VI, King of Portugal, Brazil and the Algarves (b.1767:d.1831)1) Pedro I, Emperor of Brazil (b.1799:d.1844)2) Infanta Maria Antonia (b.1801:d.1816)3) Infanta Maria Isabel (b.1802:d.1881)4) Manuel II, King of Portugal and the Algarves (b.1804:d.1879)5) Infanta Maria of the Rosary (b.1805:d.1860) a carmelite nun6) Infanta Maria Ana of Jesus (b.1806:d.1810)7) Infanta Maria Francisca of Salvador (b.1808:d.1890)2) Louis XVII & I, King of France and Louisiane (b.1781:d.1834) m. Maria Amalia of Austria (b.1780:d.1866)1) Louis II of Louisiane (b.1803:d.1862)2) Maria Clotilde of Louisiane (b.1805:d.1810) died of smallpox3) Maria Magdalene of Louisiane (b.1806:d.1848)4) Henri Philippe, Grand Duke of Saint Domingue (b.1811:d.1870)3) Louis XVIII, King of France (b.1785:d.1826) m. Maria Amelia of Naples and Sicily (b.1782:d.1821) (a) m. Maria Francisca of Salvador (b.1808:d.1885)1a) Henry V, King of the French (b.1809:d.1871)2a) Marie Louise of France (b.1810:d.1818)3a) Marie Felicite of France (b.1812:d.1889)4a) Sophie Helene of France (b.1814:d.1840)5a) Charles Louis, Duke of Angouleme (b.1815:d.1831)6a) Louis Alphonse, Count of Provence (b.1817:d.1820)7a) Marie Josephine of France (b.1819:d.1900)8a) Marie Éthiene of France (1821)1b) Marie Louise of France (b.1823:d.1894)2b) Louis Albert of France, Duke of Britanny (b.1825:d.1900)3b) Marie Antoinette of France (b.1827:d.1880)4) Sophie Beatrix, Fille de France (b.1786:d.1878)Just a family line involving an initial POD of France not losing Louisiana to Spain in the end of the Seven Years' War (with most of the population of Quebec later moving to there) and later the children of Louis XVII (including a Dauphin that lived to adulthood) escaping prison after accidentally hearing about the execution of their parents and fleeing firstly to Britain and later to Louisiana, who remained under royalist control). After a Revolutionary and Napoleonic Wars that follow similarly to OTL, the older of the two Louis declines returning to France and passes the throne to his younger brother, who becomes Louis XVIII of France
Another problem here is that the French king couldn't choose his successor, as explained here. So unless Louis XVII is killed/deposed, he'll remain King of France and be succeeded by his eldest son. The only thing he can do IMO is give up Louisiane to his younger brother, who will become Louis I of Louisiane.Portugal was never considered for Marie Thérèse (although a match between José, Prince of Brasil (Joao VI's eldest brother) and Madame Elisabeth was considered in the early years of Louis XVI's reign). Fiancées that were considered were the short-lived son of Carlos IV born in 1780, her cousin in Naples, OTL Louis Philippe (although Antoinette had a personal dislike for Philippe Egalité) and Victor Emanuel I of Savoy. A "joke" candidate was Gustaf IV of Sweden, although I seriously doubt such a match.
Not only that, but such a massive POD in 1763 (France not losing Louisiana) can alter the entire cause of the French Revolution. Why? Because do you really think that Louis XVI (who was reluctant to help the colonies from the start) is going to help the Americans here? He was only talked into it by his ministers who convinced him that it would be a show of strength against Britain. Secondly, even if he does help the colonies, if he still owns Louisiane he's in a position to force them to trade with him instead of England (he expected this OTL and was pissed off when the former colonies continued to trade with Britain instead). This trade would perhaps "alleviate" the debt incurred.
Alternatively, if he doesn't help the colonies (but doesn't help the Brits either), they need to find foreign assistance elsewhere. Spain might, but not controlling Louisiane she'd have no interest; the Dutch can provide financial backing as they did OTL, but ultimately, my opinion is that Britain keeps the colonies. The colonies' revolution fails or, without French assistance, the national debt isn't worsened (it was actually getting under control before Louis decided to get in on the action in America IIRC), France might not revolt for another few years (if at all).
And that's leaving out the idea that a king of France will willingly give up France to become king of the boonies (no offense to the Missouri-Mississippi basin of today), Brasil at least had some infrastructure when the Portuguese court arrived there OTL. Same goes for Brasil, a nerfed/different French Revolution with no Napoléon means that likely the Portuguese court never flees there, which means Brasil (hell, most of South America) doesn't get its independence the OTL way.