One important step we should take is to gather and recruit meguca with skills that may be useful. Meguca can be located by telepathically contacting Nadia, Homura, or any of our other contacts and asking them for further contacts or by asking around on the IRCs. I think we did talk to Nadia about this at some point, but she mainly brought up girls in Japan. Homura did bring up a list of girls she thought might be useful for Walpurgisnacht, but she might have omitted some less directly combat-relevant meguca.
Some types of meguca which may be valuable are listed below. We might not be able to find and recruit ones from every category, but keeping this list in mind would be helpful.
Social thinkers
If Feathers is too powerful to beat, or if it is sympathetic and should not be killed, these may be highly useful for talking it down. Mind mages are likely irrelevant, as Feathers is probably too strong to fail a will save against any of their effects, and using them will probably anger it, but social thinkers should at least be able to give tips on how to handle negotiations and may be able to participate in them.
We should therefore look for megucas who have to power to be really good at social stuff, or who have powers that make them competent at everything, and try to persuade them (via cleansing, clear seeds, material gifts, defeating bad actors who threaten them, and rhetoric) to help us. We should discuss potential avenues of negotiation even before we have a concrete idea of Feathers's capabilities and desires, discuss again after we know that negotiations are open, and possibly bring some social thinkers in during negotiations.
Social thinkers would also be useful for helping Constellation conduct diplomacy in general, minimize the angst produced by dropping the Lich/Witchbomb, etc.
Buffers
Magical girls who can boost intelligence, charisma, physical capabilities, magical strength, or whatever else are extremely useful. They can help us negotiate better, act as force multipliers in combat, and are pretty much always useful.
Precogs
It might be the case that Firnagzen does not want to deal with these in his work, but in canon, not only does Oriko have precognitive powers, but Mabayu Aki from Scene Zero and Mel Anna from Magia Record seem to have some ability to tell the future as well. While these may not be canon to PMAS, they do indicate that precognition is a valid and not amazingly rare power among Meguca in the original canon, which makes sense, as you'd expect a lot of girls to wish to know the winning numbers for the lottery, what would happen if they confess to their crush, etc.
Gathering precognitive girls would not only help us stave off potential danger, but also help us detect dead ends during experimentation, see which approaches would work best in social encounters, determine likely outcomes to battle strategies, and even serve as a source of quick and easy funds (Have Inoue Yuuki make fake identities, then have the precogs earn tons of money in those identities by gambling and options trading or whatnot). And while there do not seem to be perfect precogs in the Madokaverse, it seems unlikely that every precog's biases would be perfectly aligned, which might allow us to reduce bias and see around individual blind spots by combining the outout of each precog.
Mind Mages
While Sabrina would likely balk at brainwashing enemies, it would be highly useful to, with their consent, have a friendly emotion controller cast a calming spell on her friends when she is revealing a disturbing secret to them. This may reduce their chances of flipping out and doing something rash.
Also, emotion controllers may well be useful in dewitching efforts. As shown by the examples of Kirika's witch at the end of Oriko Magica and Saki Asami's witch in Kazumi Magica, it is not impossible for a witched-out meguca to retain or seize control, at least temporarily, if sufficiently determined. By some combination of increasing positive emotions, decreasing negative ones, and boosting willpower, it may be possible to help witches regain control of themselves, either temporarily by casting spells on them, or on a long-term basis by giving them enchanted items to do these things. This would be immensely helpful for acheiving many things.
Additionally, while brainwashing enemies may be morally dubious, brainwashing yourself is much less so. Simply increasing your energy levels would greatly improve productivity. Removing negative personality traits may be worth looking into as well.
All or Nothing Powers
If Feathers cannot be reasoned with, we may have to slay a being of godlike power. In order to accomplish this, we ought to look for meguca with absolute powers, such as unblockable attacks (ex: Sting), undodgeable attacks (ex: Gae Bolg), perfect defenses (ex: Avalon), etc. Meguca with these sorts of powers may be vital for securing a combat victory against Feathers.
Equalizers
By these, I mean meguca who can do things like become stronger when facing a stronger enemy, weaken stronger enemies down to their level, increase in strength as a fight proceeds, always win or at least never lose when certain conditions are met, etc. It would be even better if we could have them apply their powers on all of our allies. Meguca like these would also be of tremendous use when fighting an overwhelming foe.
Thinkers
Nearly any meguca with thinker powers would be useful to us. They could help us gather information both about Feathers and about more mundane threats and opportunities to help us prepare better, process the information we already have to determine solutions to our problems, manage our logistics, improve our combat tactics, and so much more.
Trumps
Meguca with powers over powers would be hugely useful in a wide number of ways. They could help break through limitations on powers, such as Sabrina's limited radius or inability to make grief constructs that do certain things, boost powers, find new applications of powers, etc.
Learning Powers
A lot of things need to be learned, such as combat skills, enchanting skills, social skills, etc. Any meguca with the ability to boost learning or directly impart knowledge would therefore be extremely useful.
Revival Powers
Wishing a beloved relative, friend, or pet back from the dead is probably common, and there might be at least one girl out there who can raise the dead. Such a girl would be insanely useful, as depending on her exact abilities, we might be able to vastly increase our numbers and gain access to powerful historical meguca, create amulets of resurrection to improve our survivability, enable riskier experimentation, etc.
Powers over Witches
Several meguca from the original works have some sort of ability to do something or other to witches, such as Sasa Yuuki from Oriko Magica (can control them) and Lapin from Tart Magica (who can retain control as a witch and revert back to a meguca). These may be of vast help in restoring witches, and investigating their capacities might yield insights into dewitching and perhaps even taking advantage of the capabilities of the witch state.
Any other hax or esoteric abilities
These may well prove useful in some way or another.
Looking for these would not have to be a hugely time-consuming process, in universe. However, characterizing dozens or hundreds of meguca would be extremely time consuming and difficult for Firnagzen. I therefore propose delegation.
Step 1: Have Nadia help us establish connections with the Red Bird Association. They apparently have many teleporters, and will thus be vital for facilitating rapid expansion. Do our uttmost to win them over, by distributing cleansing, clear seeds, money, etc.
Step 2: Ask each of our contacts for lists of all the meguca they know, their group affiliation (if they have one), as well as their powers, any notable characteristics, and (if the contact is Lich/Witchbombed) if that meguca is Litch/Witchbombed. Remove duplicate entries and consolidate the information in them (Inoue Yuuki may be able to code something up).
Step 3: Divide these lists between several people. I propose Homura, Kyouko, the Shiogama Refugees, Tsuruya Yuki, Nakano Mika, and Umika Misaki, at least at first. In my opinion, these are all either witchbombed or probably stable enough to handle it, fairly mature, and allied to us. Have each of them go through the part of the list belonging to them, and contact the girls on it, using the original recommender as an intermediary to set up a telepathic connection. Have them prioritize meguca with the powers mentioned above, and reach out to group leaders in order to recruit whole groups at a time when possible.
Sabrina and Mami can take primary responsibility for diplomacy with Tokyo and the Regius group. Even there, we would mainly be contacting the most important meguca, and not the rank and file.
Step 4: When a group agrees to meet, use a teleporter to deliver teleport Tsuruya Yuki over to claim a building as a teleportation hub. They can then travel freely to Mitakihara for cleansing, clear seeds, and gifts. If teleporters are not immediately available, then have Sabrina perform a delivery once in a while, perhaps taking a sealed grief construct in a ballistic trajectory at maximum speed.
Step 5: Sabrina can offer advice, provide demonstrations of her power, take status reports of what is going on, talk to important leaders and meguca with highly notable powersets, and step in to resolve exceptional cases. It may also be worth stepping in to bulldoze some notable bad actors, such as the Saja and San Shi Hui, to rapidly boost our reputation and increase the number of meguca willing to sign on with us, though this would have to be weighed against the chances that unprovoked expeditionary actions might make people think of us as warmongers.
Step 6: Eventually, have our first group of recruiters recommend trusted contacts that they have recruited to serve as additional recruiters they will manage.
By doing this, we will be able to recruit numerous meguca in a short time, without putting too much of a burden on Firnagzen. He could just treat most of the meguca as indistinct numbers and only model a few leaders and exceptional individuals.
