Morocco and Mauretania were on the fringes of the Roman Empire and were very much on the periphery but how could a Roman successor state emerge.
Could we see a Roman general holding the region against Berber incursions and ensuring its survival and independence?
Is this possible?
The way you've described, unlikely.
I personally love the idea, but crucially, you need to have.
1) A good relationship with the Visigoths
2) A good relationship with the Berbers,
3) Be Roman.
I actually think a 'Exarchate of Ceuta' could fit the bill here. It is ostensibly the remnant of the Empire in the West / Justinian. What I think you need is a unexpectedly successful pair of PoDs.
You need a unified Berber community/North African community. Not necessarily politically, but an alliance of sorts after a slightly more successful (for the defenders) second invasion of the Maghreb. That should be able to amass a large enough force to kick the Caliphate out.
But, I think that could lead to a major restructuring of the Exarchate so that it was economically based in the western fringes. So increased trade with the Visigoths and Britons, stronger city, etc.
Third invasion could be a rout that reorganises to halt the Caliphate at Melilla, and effectively leaves the Middle and High Atlas mountains as the border between the Caliphate and the Exarchate - the latter inviting all Christian Berbers to live in the region.
At which point your Exarchate is pratically a Romano-Berber state that can consolidate and expand its fortunes via the salt-gold trade, and based in a potentially very defensible "Constantinople in the West" at Ceuta.
But yeah, my PoD would be to have the Romans prepare the Moroccan region in case of a second invasion. Many alliances, lots of trade, beg Constantinople for gifts to give them to solidify the position.