But again the main problem is how really emotional Shiism is and the obvious problem with Taqqiyyah and how that fits within Mu'tazili thought.
That's a problem, indeed. That say, we're talking about an era where Shi'a and Sunna are relatively porous to each other
It cease to be the case around the Vth s.h/mid-XIIth , while the Sunnah as a social ensemble is already existing and doesn't have the same dogmatic and exclusive definition. It seems to give us some margin, if limited.
Roughly, the civilisational area of Islam allowed scholars (in particular, but not only) to move away within the former and to establish schools on long distances, as Is'mailism and Fatimids points (even if you had a Maghribi and Ifriqiyan Sh'ia at this point)
Now, what would push mu'tazili scholars to turn as
du'at...
Maybe a stronger repression of the Abbasids on what looks like as religious movements, due to some traumatism (nearl-successful coup tentative? Huge rebellion close to their cores?) and a huge suspicion on what looks as reformation schools?
From there, having a mu'tzaili da'i fleeing an uncomfortable political context and giving the climate of religious turmoil in North Africa, and the unability of Abassids to really act there (even trough proxies or vassals, as Aghlabids) : you may end with some syncretism happening relatively easily, with a large Mu'tzaili influence.
Tough we know, trough North African biographs, that many scholars from Eastern Islamic world went in Ifriqiya, including mu'tazili. Ziyadat Allah I adopted mu'tazila (more in order to imitate the Caliphe than out of huge inner pressure, of course).
One shouldn't forget about Ifriqiya's role into the connection of North Africa and Eastern Islamic world.
Now you may be suspicious as such syncretism conveniently happening : but
it did happened historically!
We know, trough al-Ash'ari that the Zaidi school was closer to Mu'tazilism (apparently thanks in no small part to the relations between Wasil ibn Ata' and Zaid ibn 'Ali).
Ancient texts specifically mention that Idris was both Sh'ii AND Mu'tazili, and it's likely that if he was described as part of the Sunnah, it was due to a change of definition of Sh'ia.
"Emotionalism" isn't something that was really bound to Sh'ia at this point, non in small part because Sh'ia as a dogmatic ensemble wasn't that well defined before the XIIth century. It can be more easily jury-rigged, IMO, than with more well defined schools in Bagdad.
So, rather than Isma'ilism (altough I don't rule out at all some similar syncretism), a more powerful Zaidi or a neo-Zaidi school may simply be the key, revivided trough Mu'tazili du'at.
The key problems being that this religious turmoil would make it vulnerable, that the struggle against khariji radicalism would make orthodox teaching more influents, and the limited ressources at hand in a particularily divided Maghrib.
That said, Fatimid success story points that's possible.
but how will this state survive the test of time?
Frankly, Idrisid Morroco did well until the Xth century. State organisation was rudimentary compared to what existed in Karwan, without mentioning Bagdad, but it managed to overcome periods of political division by itself.
Eventually both Fatimid and Umayyad pressure (mostly by proxy) plus khariji revolts, had to happen in the same period to end them , IOTL.
(To the point Fatimids attempted at some point to pull an Idrissid as their vassal in Maghrib against Ummayyads, as the more legit and safe choice).
Giving how Fatimids did in Africa, I'm not at all unconvinced that a neo-Zaidi school, a syncretic school, or any mix we discussed, couldn't raise as an official teaching at least in the Islamic West, and thanks to a less dogmatic ensemble than Fatimid Isma'ilism, couldn't have a more lasting influence (that IOTL Isma'ilism did, nevertheless, even if fragmentary and "hidden")
