Jon and Daenerys's Baby Could Unite Westeros
Both Jon and Daenerys would have to win the political support of the southern Kingdoms in order to unite Westeros. Their incest (and the child born of it) would become public record, but Cersei's love affair with her twin brother Jaime is now also publicly known. It's hard to tell how much the people of Westeros are willing to tolerate incest from the highborn, but it's now unavoidable since the two camps claiming the Iron Throne engage in it. Cersei Lannister has political support, but this is mostly out of fear and intimidation. Cersei is actually hated throughout the Seven Kingdoms, while Jon and Daenerys could bring stability to Westeros at last if they claim the Iron Throne, with a Targaryen/Stark scion to continue the legacy.
However, happy endings are not Game of Thrones' style, and all of this is moot if the living don't defeat the Army of the Dead first. We don't know how long the war against the Night King will last, but it's possible it won't span the 9 months necessary for Daenerys to give birth. The longer the war rages, the more likely Daenerys will be forced to retreat to the relative safety of Dragonstone (where she can subsequently be attacked by Cersei and Euron Greyjoy's Iron Fleet). A pregnant Daenerys has powerful enemies threatening her life no matter where in Westeros she is, and she will require more protection because she's at her most vulnerable - taking valuable assets away from the war itself.
Related: Why Cersei Needs The White Walkers
Of course, it's likely Daenerys isn't pregnant at all. The Jon/Dany story is already complicated enough without throwing a baby into the mix, especially in a war against ice zombies. Meanwhile, the other Queen of Westeros is certainly pregnant. The rumors about season 8 may well be about Cersei Lannister.
Who Is The Father of Cersei's Baby?
The rumor about the possible Game of Thrones pregnancy story is said to lead fans to “probably make an immediate assumption about the identity of the baby daddy" and that it would be “both dramatic and darkly comedic.” While the latter would apply to Jon and Daenerys, there wouldn't be the question of who the father is, unlike there could be for Cersei's pregnancy. Fans did see the Queen and her twin brother have sex before Jaime left to conquer Highgarden in season 7. However, Cersei has a history of multiple lovers and it's much more possible that this time, Jaime isn't the father of Cersei's baby.
In a surprise twist, season 8 could reveal Cersei is pregnant by Euron Greyjoy. After all, Euron openly lusts after Cersei. The ruler of the Iron Islands was in King's Landing right before Jaime left for Highgarden. Fans could learn that, unbeknownst to Jaime, Cersei did give Euron the reward he covets for delivering her Ellaria Sand, the murderer of her daughter Myrcella. This wouldn't be the first secret Cersei and Euron have maintained together; they already faked Euron fleeing from King's Landing at the sight of a White Walker at the armistice, only to reveal that Euron was sailing to Essos to being the Golden Company to Westeros to fight for Cersei. The alliance between Cersei and Euron could also have been consummated, to Jaime's chagrin when he finds out.
Jaime is already disgusted by Cersei and the Lannister twins are now estranged. With Jaime fulfilling his pledge to go North and fight in the Great War, Cersei carrying Euron Greyjoy's child fits the bill as a "darkly comedic" possibility. Jaime has gradually transformed into a fan favorite, while Cersei and Euron steadfastly remain villains we love to hate. A surprise union between Lannister and Greyjoy would only increase Game of Thrones fans' desire to see Daenerys and Jon finally place a Targaryen on the Iron Throne.