-Theros: I haven't had a chance to read Godsend yet, but from what I understand, Elspeth killed Xenagos, but was then speared by a fearful Heliod and sent to Theros's Underworld. The pantheon returned to its 'original' nine, Daxos was killed and Returned, and Ajani planted a seed of atheistic revolt in the people of Theros.
-Gideon: Gideon sacrificed his life to defeat Nicol Bolas but was reunited with his Irregulars in a Heartwarming Redemption that makes no sense with what we currently know about the Therosian afterlife. Also died with hanging plot threads, specifically with Chandra.
-Chandra: Haven't read the comics yet, but from what I've seen, Chandra is taking Gideon's death hard. Also confessed her feelings for Nissa which were probably reciprocated, but the book is poorly written so it's not 100% clear.

-The Pantheon: With Xenagos dying in Nyx, he should be permanently dead leaving a RG sized hole in the Theros Pantheon. While the Gods my be okay with this arrangement, Magic players abhor an incomplete cycle.
-Elspeth: When Elspeth was given the axe (or the spear), the Creative team made it very clear that her story wasn't over. We were given a teaser showing us Elspeth's death mask and more recently, Elspeth, Knight-Errant became a masterpiece depicting Elspeth in the Underworld holding her mask.
-Death and sparks: According to Doug Beyer, sparks generally don't survive death. This presents a problem for bringing Elspeth back, unless they want to apply a caveat (which the masterpiece would imply).
-The Underworld: When someone dies on Theros, they are taken to the Underworld, the sunless, nightless realm of Erebos, whom Heliod banished out of fear. I don't know if the exact means of transport have been established (what happens if you get eaten by a hydra, for example).
-Hall of Heliod's Generosity: In Modern Horizons, we were introduced to a new Legendary Land depicting the stronghold of Theros's light.
-Ravos, Soultender: In Commander 2016, we were introduced to Ravos. According to the Theros page, As an oracle of Athreos, the god of passage, Ravos knew the fleeting nature of life. Even so, watching his beloved perish in battle was too much for Ravos to bear. He bargained to help Athreos guide the dead to the Underworld in exchange for seeing his dead lover once each year. Now, Ravos plots his beloved's rescue, and secretly defies Athreos by escorting the dead back to the land of the living.
-The Love Triangle: In Kaladesh, Chandra has the (I will never not use this pun) hots for Nissa while Gideon (from Theros) is in love with Chandra.
-Ajani: Ajani has obvious connections to Elspeth and features in Chandra's comics. He also has an interesting parallel to Chandra in that both have lost very close friends with Theros connections.
-The Immortal Sun: In the War of the Spark novel, Chandra had to activate the Immortal Sun and felt it's pull on her. The Sun holds the spark of Azor, parun of the Azorius Senate and uses sparks as power (Chandra used her spark to help fuel the sun when she turned it back on). While the planeswalkers debated whether to destroy it or use it as a weapon, the artifact is currently still around.

-Heliod, Erebos, and the Afterlife It would be an understatement to say that Heliod and Erebos have a bit of a rivalry. While Heliod may send champions like Gideon to thwart Erebos, Erebos gets them all in the end. But what if this wasn't the case? What if Heliod, in his generosity created (or coopted) a place where those that served him could be spared the fate of all Therosians? What if the Hall of Heliod's Generosity has a double meaning; as both the source of Theros's light and a place for Heliod's fallen to rest? This would explain how Gideon could find himself in a sunny afterlife surrounded by his Irregulars without being a hallucination brought about by his burning brain stem, thereby undermining the heartwarming part of his Heartwarming Redemption.
-Chandra and Ajani on the Road to Hell: Ajani and Chandra have both lost dear friends. But what if there was a way to bring them back? Would you risk it all for one more day? It seems plausible that Wizards could want to tell an Orpheus and Eurydice style story with Ajani being Orpheus and Elspeth being Eurydice. Ravos could also play an important part, seeing how he is secretly bringing people back from the Underworld.
-The Tenth God: I find it interesting that the two people that Chandra is closest to have connections to Theros...sort of. Gideon is obvious, but Nissa doesn't really have a connection. However, Chandra and Nissa together represent the missing color pair in the Theros pantheon. I don't know how Creative would tie it in, but I have a theory that I don't think will actually come to pass. In order to bring Gideon back from the dead, Chandra might have to give up something very important to her. Maybe the most important thing. The essence of her love for Nissa could then become the basis for a new RG god. This idea is more wish fulfillment than anything but I'll explain why I don't think it will go down that way later. However, Chandra and Nissa colors completing the pantheon feels too perfect to be coincidence.
-The Greek Tragedy Let's assume everything goes well. Ajani manages to get Elspeth back from Erebos, hell, maybe Erebos is oddly cool with it, and Chandra brings Gideon back to life. Now our heroes are gearing up to leave Theros behind them. Just one little problem. Gideon and Elspeth don't have sparks anymore. They're stuck. All of our heroes efforts are for naught. They may be back in the world of the living, but it is only temporary. Erebos claims everyone in the end. Elspeth doesn't take it well. She's spent far too long in the darkness and her biggest reasons for staying (Daxos and a permanent home) are gone. Then Chandra comes up with an idea. We have an extra spark laying around; the Immortal Sun. We can give Elspeth Azor's spark. That leaves two problems however; Gideon is still s.o.l and the Immortal Sun is now useless. So Chandra comes up with another idea, one she isn't thrilled about. She can loan her spark to the Sun on the assumption she'll be able to get it back later (it's basically what Ugin was going to do with Azor anyway before that went sideways). Elspeth and Ajani are able to leave Theros (probably to go fight Phyrexians) while Chandra and Gideon get some much needed closure while she is on temporary hiatus.

-Gideon's arc was a mess in War of the Spark. He never got a proper confrontation with his backstory (Chandra defeated Baral, Nissa 'trapped' Emrakul and killed the other Titans, Jace got his memories back, and Liliana put Josu to rest and got out of her contracts. Gideon never saw Erebos again except in his worst nightmares). Additionally, Gideon's relationship to Chandra was basically ignored after Hour of Devestation. Tying up these loose ends would make his death much more palatable.
-Phyrexians are on the short list of threats to the Multiverse. With a return of New Phyrexia, we'll probably see a return of Koth. Creative might want to pull back on Chandra's involvement in the story so she doesn't steal the spotlight from Koth.
-If Wizards know they are "killing off" Chandra soon, it would make sense to give her a last hurrah in the core set (similar to Nicol Bolas last year). Additionaly, Chandra's Regulator could put constraints on what future Chandra's can do; unless they aren't printing anymore for awhile.

-Chandra is basically the face of Magic now. There is no way in hell Marketing is going to let you kill her off.
-You think they're going to build up a relationship between Chandra and Nissa for three years just to throw it away? I mean they did it with Gideon.but that was a straight pairing.
-Bringing Gideon back would fly in the face of what Nic Kelman said in his polygon interview promoting Signature Spellbook: Gideon