Initially, the primary motivation for changing the map from v3 to v4 was so that more kingdoms would be able to fit. Unfortunately, it seems this motivation fell off the radar, but the map change went ahead anyway.
One interpretation of what Sirrian has said about new kingdoms is that the only way we’ll get a new kingdom is through their “country inspired” mechanism. In the past, this has (apparently) seen new kingdoms added when new languages are added, with the new kingdom themed after the new country (Shentang and China are the only example I’m aware of, because I’ve only been playing a year and a bit).
Sirrian did also respond somewhat positively to the idea of adding an Australia-inspired kingdom. This seems unlikely, because of something we call “cultural cringe.” Essentially, most Australian companies on the world stage have something of an inferiority complex that means they tend not to promote Australian-themed products, and sometimes even to hide their Australian roots. (This is not an accusation; it’s really, really common.) In our devs’ case, this has meant minimal Australian-themed content (eg: no Australia Day pet), but they certainly haven’t hidden the fact they’re Australian based. So we may yet see an Australia-themed kingdom at some stage. Personally, I feel many countries respond very positively on learning that our devs are Aussies – but I’m not really in a position to judge.
Finally, the other way we might get a new kingdom is if Sirrian has a story idea that requires one. He explicitly mentioned that he’s intrigued by what might be found on the island between Whitehelm and Khetar. Sadly, I suspect any such quest line would probably be trimmed down to a Faction, in the current state of the game.
My final conclusion? We might get a new Kingdom if a new language releases, or if Sirrian comes up with a story that must be told. But it’s unlikely to happen before every Kingdom has a Faction.