I agree that Mara needs a girlfriend, which would have to be a new character, perhaps one of the other prisoners who later goes with Mara and lives with her at the estate. For Berenice, otoh, I could imagine interesting scenes with both Caenis and Dorion. I know Caenis has a negative attitude about her in the novel, but there's no plot reason for that, and she could be both realistic about any official Titus/Berenice marriage being negative PR for Titus and interested in Berenice as a person. (After all, Caenis herself never married Vespasian despite being his life long companion - btw, Caenis is a historical character, not invented, she and Vespasian really were a life long affair.) They also are on different ends of the social scale, with Caenis the freedwoman of Antonia Minor, i.e. a former slave, and Berenice a princess, and yet both in completely different situations from where they expected their life to end up.

As for Dorion, given that Berenice is both a Jewess and deeply imprinted on Greek culture, and, again, a Princess, I think Dorion in the volume 1 era would both want to meet her - since Joseph uses Titus as a go-between to talk Dorion into marrying him, it could be worked into the plot easily - and be confounded by Berenice being both. Considering Dorion's Antijudaism is very much tied to her marriage getting toxic which doesn't happen until mid volume2, I'd be curious to see how she would handle a woman who has similar and yet different identity clashes as Joseph and whom she can't dismiss as being her social inferior or as not knowing about the visual arts. (Rather the contrary, for Berenice as an Herodian princess.)

Another thing about Mara: simply by adding some scenes from her pov throughout the saga, we could see her development from shy and broken war captive to confident estate manager (who still loves Joseph until THAT event in volume 3, but doesn't depend on his presence anymore in volume 2 already). Since she's in Rome during part of volume 2, I'd give her the chance to spit on Vespasian's grave, so to speak. (Since the Romans are cremated, it would have to be his statue once he's a god, I guess.) And you could give her a daughter in addition to the two sons (which have to be there and have to be sons for plot reasons) and show us the two of them relating to each other. I'd also be curious how the Jewish community in Rome relates to her, since her being a former war captive and her original marriage with Joseph was a big issue in Israel and in Alexandria, but things in Rome are somewhat different.

The one character where you basically don't have to change anything is Lucia. :)