One of my favourite fanfictions ever is Dæmorphing, by Poetry (@featherquillpen on tumblr, hopefully you don't mind the tag). And I have given some thoughts as to the question why I liked it so much, since this could give me a hint as to what to look for next in my fics. I don't really have a definitive answer, but I do already have a couple of ideas.
There are two basic points I think may have been essential to why I liked Dæmorphing: one, how well it presented the non-human species and gave them characteristic, helping me to see the personhood of even the Yeerks; and two, how well it fuses the worldbuilding of His Dark Materials into Animorphs, almost seamlessly, making me think that in hindsight, the implementation of this element from HDM seems the most rational thing.
The former of those gave me the beginning of an idea for a recursive fanfic about Mulefa living on a different planet in the Daemorphing universe. Currently, this idea is going nowhere, and technically I didn't get greenlight from Poetry to publish it even if it was already written. So for now, I'll just put this aside. However, some weird thoughts about the nature of fusion fics led me to think of a fanfic that will do the reverse of what Dæmorphing is - instead of being Animorphs with HDM worldbuilding inserted, being HDM with some Animorphs details implemented. Is this possible? I don't know, let's see how many random nonsensical ideas I'll manage to pull off.
By nature, His Dark Materials is a story about the loss of innocence. It also has a huge battle-at-the-end-of-days, but it's explicitly background. There are Good and Evil sides, even if they're flipped from the traditional religious view (think demiurge, only he didn't actually create anything). And while both sides have proven that they aren't as Good as they claim to be, I think there is clear bias towards Lord Asriel's cause (though I don't think it's the most interesting thing about these books by far).
There is also an inherent difference between Animorphs and HDM in that Animorphs is serialized. This allows a start filled with a couple of separate episodes that only slowly grow to an overarching series. Also, Animorphs has a very clear endgame: the series must end with the defeat of the Yeerk empire and saving Earth, and the ending arch has to start with the Animorphs identities being found out. Or, well, you can drag it a bit more, but those are essential events. HDM, on the other hand, has more of an element of mystery ("eventually she would know more about Dust than everyone in the world", indicating this is to be Lyra's journey; but also the GOBblers and what's going on with them; who is the man who stole Lyra's Alethiometer and what is the Subtle Knife, and what is the prophecy about Lyra), though it doesn't have one clear quest throughout.
Lyra's quest starts with wanting to see more of the world and learn about Dust; then it becomes saving the children from the Gobblers; then she wants to go see her father and give him the Alethiometer (which she started assuming along the way was her main quest all along), and if it means overthrowing the King of the Armored Bears so be it; then she decides her quest is to save Dust, because all the wicked people hate it. That summarizes her quests for book 1 alone, but doesn't really summarize the book itself, but maybe we'll start with this odd concept: Dust is, after all, central to the plot of the book, and if Dæmorphing managed to insert severing of dæmons into Animorphs... it can work.
So... random ideas:
The Master of Jordan College and Lord Asriel are both the equivalent of Andalites. Are they actually Andalites? Are they maybe regular humans from a super advanced society or something? Am I speaking rubbish? Well, I think I can answer the latter myself, thank you.
Mrs. Coulter and the Oblation Board are a cover to the Yeerk invasion. Possibly the Magisterium at large is - though it's too powerful an organization, so it's probably only infiltrated by Yeerks.
The Master either gives Lyra an Escafil device in place of or alongside the Alethiometer. Either way, in this universe the Escafil device has much more to do with Dust.
How is that? Well, perhaps it functions on swapping the human and their dæmon. After all, the reason a dæmon settled as a predatory animal isn't that much of an advantage in a fight is because they can't attack humans. They can attack other dæmons, which is a power unto itself I suppose, but of only one side has access to this swapping technology...
That would probably mean it's especially useful for children, which I think might work with the overall themes of HDM - though there might be another difference, where even if it's not easy and is done differently, it's possible for an adult to relearn how to use it.
It may also be required to serve as the same sort of plot device that the Alethiometer was in the original HDM; maybe something about this swap allows one to read things in the Dust or something, if the usage is as limited as the Alethiometer's was in the original it could also be interesting.
The Gyptians don't necessarily know about the Yeerks, but they know something smells.
The severence is done in order to weaken children to be future hosts; it works only partially, and not well.
The Armored Bears stay the same, maybe Iufor is a voluntary host or something, though that would create a host of more problems.
Lord Asriel was not infested in captivity. If you want to add the Abomination anywhere you'll probably have to consider deeply who can randomly become an Andalite and whether this actually makes any kind of sense.
Both sides are bad is something both Animorphs and Northern Lights/the Golden Compass agree on, to a degree. I think equating Lord Asriel's side to the Andalites doesn't work becuase the former purport to fight for the betterment of humankind without sarificing more than one civilian. The Andalites are willing to completely destroy Earth to defeat the Yeerks. It's true that both Asriel and the Adalites supposedly focus on the big picture, but... well, actually Asriel was never persuaded that the problem on his own world is something that needs direct attention. And in the end, building "the Republic of Heavens" is more of a spiritual struggle than a physical one. There are quite a few differences.
This should probably have not seen the light of day.
Thank goodness it's night here and I'm bored enough to actually publish this.
Thank you for reading if you managed to get this far.