Unless you are specificially looking for polities/institutions that are continued from the WRE itself
Then again, most of western kingdoms recieved the imperium at some point, from "might makes right" Vandals to Frankish kingdoms (which is basically an inflated foedus). At their core, they are creations from WRE that carried on after its collapse.
(Britain being, as always, its own special snowflake, but less so than commonly tought)
I think you'll see them seeking to establish themselves as rulers, or locations of significance to the new powers at play
Isn't what happened for most of isolated places? I mean, as we're talking about Ceuta (if we are to use contemporary names, it should be
Septa), their rulers were quite independent after a while and payed mostly lip service to the exarch or Goths, at the point the local ruler was described as king by Arabo-Berbers.
well placed to try and become a major naval force
I won't be so sure : Septa was really excentric, without much hinterland or ressources to build a fleet (as in large access to wood to begin with) and under pressure of Mauri as all
late Antiquity Africa.
Mostly, the time wasn't to small cities as economical powers : you regularily ended with de facto autonomous cities (interestingly more in episcopalian cases than municipal, as Provence's patrices) but they depended too heavily from military assistance to really pass as independent while military role was distinctive of political power since the IIIrd century.
Any post-Imperial or late Imperial power in the West have to carry an army with it in order to be relevent, would it be only
militiae, and having at least some basic legitimacy.
That said, a rump WRE managing somehow to prevent being sattelized or absorbated by ERE is always
an interesting debate.