First.
Incubators, their goals, how we figure into it.
Right now, there are two possible scenarios for what Kyubey would like to do with Sabrina:
1) Kill her
2) Witch her out
Second is pretty much obvious, considering it's his long term goal for nearly any Meguca. That's his job, more or less.
Things get interesting here if we assume the first. Why should we?
Three reasons, one of them hypothetical. We'll start with it
1) The Incubator is able to see the Potential. For our purposes, it means it's able to see how powerful a Witch would result from hatching the Meguca in question. He has a lot of circumstantial evidence to extrapolate that Sabrina's Witch is an ontological threat, same as we were told by Firn. The evidence is as follows:
Our Wish and it's result is unprecedented
He isn't able to gauge our Potential precisely, but he knows it was enough to wish for controlling Giref and
succeed, something that hasn't happened ever by his admission and by our knowledge of the setting.
He knows that the Witch gets abilities connected with the Meguca's Wish and their nature.
If he extrapolated from the following, he would be at least a little wary to Witch us out... I'd say, but nah, he knew Madoka would destroy the world in seven days and actively encouraged the outcome.
So here's the second reason
2) Sabrina is too emotionally stable at the moment. The only signifant lever he has on her psychological condition is Mami, who is already less emotionally compromised than she was by the start of the anime. Basically, to Witch Sabrina, he would have to Witch Mami first. While Sabrina is alive and Unwitched, it's practically impossible if not for the support she provides to the group, then for reason
3) Sabrina controls Grief. She's able to suck the Grief out of her Soul Gem, your Soul Gem, their Soul Gems, everyone's Soul Gems! Give me your Grief, bitchez!
Grief spiral loses its meaning near to Sabrina. It's unpleasant. It's absolutely horrifying to live through. It also doesn't accomplish the Incubator's goal.
Considering that Sabrina stabilizes the Mitakihara group with her presence, resolves ( some ) of their issues, is difficult to Witch out in the immediate future and has a real chance of winning against Walpurgisnacht together with Homura, something Kyubey wasn't willing to bet against even when Homura had only Kyouko as a party member, because Homura is a SSR support from another dimension, I would suggest that the Incubator's goal is to kill Sabrina, like he did away with Kyouko, for example.
The most likely scenario for this is an attack by a large hostile Meguca force who knows our strengths and weaknesses and has a clear counter to our abilities. That would be either the much advertised Iowa Battleship or a conglomeration of several groups from Tokyo.
I'm betting on Iowa for the immediate danger, considering I believe they are kinda on his payroll. Their existence is beneficial to him, because it invites chaos, promotes suffering, causes destruction and Wishes as a result, and is somewhat useful when he needs to manipulate someone to kill an uppity Guca with too much Potential and too little sense.
Now, there are several handles on other members of our group. A question was asked: why doesn't he use them?
I have a hypothesis.
I anime, Kyubey slowly introduced issues and tension to incite conflict almost at leisure, because frankly, PMMM cast consists of people who seem to compete in a marathon of death and Witchouts. His only goal was making Madoka contract and he accomplished it by preventing anyone from helping Homura. Now, the situation is a little more complicated, in his opinion. The new factor has outside knowledge, is here to help by her own words, prevents Grief Spirals and conflicts... almost like someone created her for this very purpose. His most likely effective strategy? Hit her with everything at once. Wait till she makes mistakes and enemies. Coordinate their attacks. And
then introduce issues. Separate the group if possible. She cares about other groups? Attack them too. Use several platforms to Witchbomb everyone at once, if that's what it takes.
The nearest possible moment for that?
Soryuo Twins.
If he can somehow convince Iowa to arrive with them.