#subsumption

Regardless, given this main character, let's first ask — what are her goals, and what challenges is she likely to run into regarding them, given her starting circumstances? This is really important, as without either of these, SI and OC-focused stories oft end up crumbling to their own lack of structure and dying four or less chapters in. They don't need to last, and they can pick up more later, but there has to always be some sort of theoretically achievable outcome they're working towards, or they're going to feel like they're drifting through the narrative without any real purpose. SIs have this easier than OCs, since it can be as simple as "survive past the canon ending", "keep this one specific character from dying", or (as I commonly go for) "find a way to escape/reverse this situation". Ashtaroth's immediate goals end up being “survive getting away from the girl who caused this”, “return to being a magical girl (if possible)”, and “figure out if any other witches are like me, as I really need to be sure I haven't been killing thinking beings all this time”.

Now let's take a moment to daydream. Irrespective of how any of it would happen yet, how might Ashtaroth theoretically interact with other basic magical entities of the world?

Other witches — are any of them sapient, sentient, or capable of communication, and how would they react to Ashtaroth's attempts to determine such? Witches live in barriers, isolated pockets of reality, so how might those interact when they come into contact with each other? Are there any witches it would be specifically interesting for Ashtaroth to clash with? What about their familiars?

Magical girls — are there any that would specifically take issue with Ashtaroth's existence? Any that are particularly likely to come in contact with her given her own choices? Any whose personalities would be fun to bounce off each other, especially if they never met in canon, or whose powers would interact interestingly with hers?

Kyubey — he's not technically magic in and of himself, but he's shown interest in “Irregular” beings in canon, so he'd likely show such here as well the moment he becomes aware of Ashtaroth. How do they interact? Also, since he has information that's simply not available from any other source, what could lead him to disseminate that, and would the result be interesting?

With that done, let's now ask what could go wrong in the process of Ashtaroth attempting to complete one or more of her goals. In fact, for me, a lot of early plotting is deciding that one thing is simply going to happen and figuring out how to make that occur (you may recognize this as a smaller, faster version of the “I planned my whole fanfic around one scene and now have to extrapolate backwards to get there” method), as well as working out what might follow and come about from that.

Mind that I am still entirely in the planning phase at this point. I've likely written down a bunch of loose thoughts and ideas in shorthand, but it's usually only after I've come up with a basic structure for one “arc” and know at least vaguely where it could go next that I'll decide “yeah, I think this has legs” and start working on a story proper. It doesn't need to be nearly as detailed as all this, you just have to have some idea of where you're working towards and what scenes are likely to occur on the way.

Once you have this basic set up established, you can much more easily extrapolate outward. As I'm very fond of SIs and butterflying canon plots, this is especially easy for me, as Character!me can attempt to work out what impact my prior actions may have on the plot as I know it, and how I may now need to react to prevent problems, easily setting up future plot threads. You can also now start exploring unclear or fringe cases the reader might also be unsure about — Subsumption starts pre-canon, so are there any witches who aren't witches yet, and could be met while still in magical girl form? If Ashtaroth simultaneously inhabits both her own body and the bodies of those she's subsumed (something only revealed later), what happens if her main body is killed or otherwise indisposed? Once we know what happens if Ashtaroth subsumes a witch, and what happens if she subsumes a magical girl, what happens if she attempts to do so to something similar that constitutes neither (Uwasa, Puella Malefica, etc)? These are often the most interesting plot threads, so I consider this important, and exploring them can let you weave in other ideas you previously spitballed about. To set up the “what happens if Ashtaroth's main body dies” mini-arc, I dipped into the “what characters would be fun to see Ashtaroth clash with” idea bucket and found Oriko and Kirika, the former of whom would naturally be concerned about Ashtaroth due to her foresight and could take steps to execute exactly that. The longer a story goes, the more chances there will be to interweave such ideas, so even if others have to be dropped due to plot developments, you'll still have some pretty interesting stuff.

Another note, solely because this is focused on SI and OC fics: some problems should absolutely not be easily resolved. Sounds obvious, but fanfic often falls down here. Letting problems be longer-term issues lets you set up overarching plot threads and goals, which gives the characters challenges to overcome while also letting you explore other ideas further in the process. Switching fics for a moment, in Most Omnipotent Goddess, I theoretically could have written it so Jubileus!me could just snap my fingers and fix the Translunar Faith Allocator in an instant, as the limits defined by canon are loose enough I could claim nearly anything. By not allowing that immediately however, I get to use the process of attempting to resolve the issue in future chapters to write some cool scenes (ex. Reforming the moon by hand, the meeting with the Auditio), and dovetail it into other plot threads that let me explore new areas of the world's cosmology (ex. the Empyrean → Inferno → Sheba), and things that were never touched on in canon. Sometimes I would actually consider this a weakness of mine, as my desire for my protagonists to have significant obstacles; to struggle to deal with them before eventually triumphing and having grown in the process, can leave them feeling weaker or less capable than I actively intend. I don't think this would be as much of a problem if I wrote faster, but it's lead to criticism in the past that still weighs on me. Nonetheless I think it's a very important aspect of plot structuring that a lot of fanfics and amateur fiction just sort of ignores, which is what's led to the glut of terrible and quite impatient isekai we're still suffering through.

...I'm no longer sure this is even answering your question. In fact I think I might just be rambling at this point, and I've made it sound so, so much more complicated than it actually is. Uh, basically:

Make sure to theorize and explore as many potential plot threads as you can early on, so you can cherry pick the best ones and neatly tie them together later.

Make sure your protagonist has at least one clear goal from the outset that they can at least attempt to work towards.

Do not make things too easy for said protagonist; challenge and struggle is important for both character and plot development.