For "control of straits/strategic locations", there's always moving the capital away from Rome to Carthage (oh the heresy!) for control of the Strait of Sicily. Although as I've also mentioned Tingis/Tangier, I think two African cities serving as "Rome" would be excessive, so you could put the other anywhere on the Iberian side of Gibraltar from Cartagena (two Carthages, nice) to Huelva.
Not many cities have such a strategic location as Constantinople, which inherently limits the potential sites.
But for controlling strategic straits, Tangier/Tingis is possibly the best location, as it isn't hemmed in by mountains like, say, Gibraltar (or Ceuta) is and has more potential for growth.
For the purposes of this thread, I would assume the closest major city to the straits of Gibraltar would be what the OP is looking for. And it was the most important city of Mauretania Tingitana (and probably impossible to beat in the Roman Era for that province no matter how strong Mauretania Tingitana could ever get, including "Rome conquers the rest of modern Morocco"), which was part of the Diocese of Hispania in the late empire, and thus would fit as an important regional center of Hispania.
Wow, that's over 730 billion people in Iberia alone. I hope they took apart all the mountains to convert into apartment blocks with 3D printing or something somehow. Must be the future of the most wankish "Roman Empire industrialises" scenarios.