Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
Demon spells
The killing spell (Qual) and the mind control spell (Aura) are different from the Avada Kedavra and the Imperius curse in Harry Potter; since the aim is similar, it is possible we will see also the equivalent of a Cruciatus curse. This kind of spells are logical in a war context, specially one with demons involved. Like necromancy. These spells are inferior to a Dragon Slave (a magical nuke). However this is a relatively quiet story and there will be no equivalent of a Dragon Slave or of Decreation magic.
Frieren's party will encounter and defeat the demon that attacked Stark's village, and Stark will obtain his gigantic axe
Stark will see his brother Stolz again when they reach heaven
Frieren's party will meet a new generation's Himmel, at which point she'll leave the party to let them adventure on their own
Once Frieren meets Himmel in Heaven, she will choose to stay with him
Witnessing Fern and Stark's romantic development will help Frieren realize that she loved Himmel.
It's still ambiguous as to whether or not Frieren viewed Himmel as an important friend or in a romantic light, but witnessing the
Ship Tease between the kids will allow her to figure out her feelings. And when her party reaches Heaven, she'll finally be able to tell him that she loves him.
Elves are doomed to always have relationship problems, not just from a human timescale point of view, but even among their own kind
It's discussed that elven sexual drives are so low (due to being insanely long-lived) that they are now going extinct, and because there's not much biological motivation for it they don't seem particularly interested in the romantic ideal, either. (What's the point, when being too attached to any of the shorter lived races would lead to inevitable heartbreak?)
No babies means less of a need for family-type bonds, too, though Frieren is now willing to give those a shot thanks to Himmel giving her a taste of True Companions.
Friendship-type bonds can happen, but given Frieren's extremely niche interest she's not likely to find people who share her particular hobby (which may be another reason why she is willing to let Fern travel with her, as Fern is at least interested in learning magic as a means to and end and helps keep Frieren motivated on her quest).
Fern will end up being the most powerful Human mage since Flamme
Frieren and Serie sometimes make offhand and veiled references to how the "Time of Humans" is coming. When magic will advance beyond what the Elves could do in the Mythic Era and even beyond the ability of Demons. They hint that humans will end up surpassing even the elves in magical abilites. Fern will be the first and greatest of this coming era. She'll end up as the greatest mage since Flamme, more powerful than even Frieren and Serie, and usher in the final age when humans dominate.
Frieren is a demon who Flamme disguised in order to create a demon kind who can empathise.
Frieren was shown to be quite emotionally cold before meeting Himmel.Schlacht mentions that Frieren is critical to the survival of the Demon race.Flamme set up the plot for Frieren to go north to the demon king's castle when she gains the capability to empathise.Flamme herself made mentions to the origins of the Demon race, but none to the Demon King.
Elves do have adequate sex drives for their species, courtship simply takes so long that Frieren did not see any of it before she lost most of her people.
After all, at one time there were enough elves that Flamme did not consider a whole village of them to be that unusual a sight. So their drive to reproduce was more than adequate due to how rarely any of them died. It's simply that the loss of most of their kind due to demons meant that they had little capacity to rebuild their numbers, not helped by how the remaining elves seem to have all taken to living apart from each other. Frieren taking the better part of a century to fall in love with Himmel may well have been the equivalent of "love at first sight" true love by elf courtship speed standards.
The Goddess of Creation was just a very powerful Savant-like mage who died off without leaving a comprehensive system of magic for her followers
One of the ongoing themes of the manga is the notion that cultures tend to forget or distort people and events that take place a long time ago, this is especially prevalent in the Northern Empire arc, where Flamme is depicted as a man in memorial statues, while Himmel is depicted as a gruff older man. It is, therefore, not a stretch to imagine the Goddess that the people worship might've just been a mage who, unlike Flamme, did not try to systematize and rationalize her magic, and instead, left instructions in a series of stories for her followers to decipher and interpret. This also could be seen as commentary on how ancient knowledge in real life is sometimes expressed in somewhat esoteric texts (here representing the Goddess' Magic, locked away in metaphorical and ciphered texts), in comparison to modern-day's relatively straightforward and mundane scientific process (the Grimoies that Frieren collects)
Schlacht and the Demon King's plan to ensure the survival of the Demon race involves eventual coexistence.
Realistically speaking, there are only two ways to ensure that the Demon race survives in the long term. Either they wipe out humanity, or they finally figure out some way somehow to coexist. The greatest chance of the former happening died with the Demon King, and with the world entering a human-dominated era the odds of it ever happening grow slimmer by the day. That leaves coexistence. And while the Demon King may not have been able to figure out how to do that himself, with Schlacht's seer abilities they need only select for a timeline where a Demon
does eventually figure it out and spreads it to enough of the rest of their kind that it ensures the survival of the species. In fact, this may be the source of the Demon King's interest in coexistence in the first place. Rather than any true interest in coexisting, he may have merely sought the survival of the species and pivoted to coexistence as a means to an end after discovering that attempting to wipe out humanity was doomed to fail.
