Interestingly, aside from Runemaster and HOI4, PDS is actually working on an as-yet-unannounced game known so far only as "Project Augustus." It's not clear what exactly this is, and the responses from PDS devs (expected of) have been vague at best and contradictory at worst. But they do seem to have the resources to pull off Vicky 3, and I really hope that's what they're doing.
Project Augustus? Like Augustus Caesar? I heard Shams Jorjani of Paradox was seem wearing a Europa Universalis: Rome 2 shirt at their recent conference. No announcements about it, just Half-Life 3 style teasers. So who knows. But "Project Augustus" would seem appropriate for a Roman theme.