One of the reasons why I think Vicky 2 sales is underwhelming compared to other ones is of course that right now PI recent games are more popular. All popular iterations got a new version (EUIII -> EUIV, CKII -> CKIII, Hoi3 -> Hoi4), and on top of that they have a new succesful IP Stellaris and also gave new life to the Rome iteration. Vicky II is graphically already a dated game. But the most important reason is that Vicky II is incredibly complex and hard to learn as well as hard to master, even compared to other PI games. It's the one game i haven't been able to have a grasp on, because it is so hard (similar to Kerbal Space Program-difficulty). One of the very few games that are way too hard for me personally even after hours of dedication. PI has made their games more accessible. It's what HoI quite succesful, on top of offering more "alternative historical possibilities", whereas HoI2 was more realistic and had less options. I'm not saying Vicky has to follow that route or that it has to be a hard game, but if done in a similar way as EUIV, it could be a top game seller i believe.
If i never knew about Paradox Interactive or their games, and you told me there were strategy games about the second world war (more focused on warfare), the medieval ages (but more focused on family), one about Rome, one about the Victorian age, one space grand strategy game and one centered in the era EUIV covers, i would guess EUIV would be the least popular one honestly followed by perhaps the medieval age, while I would think the Rome, the WWII and the Space grand strategy game would be the most popular. It still stuns me that the EU IP is the most popular one, but it is one incredibly well made (even if you disagree with their post-content or marketing strategies, the end product while very expensive, even though during sales it's less expensive but the end product is fantastic). I believe they can do that for Vicky too.
And EUV would be a risk or Stellaris II since i'm unable to see what they could improve in EUV what they couldn't do in EUIV. CKIII had the benefit that CKII perhaps was graphically dated, and that indeed it seems things could indeed slightly be improved but the graphical change is a notable change. But for a possible EUV i'm unable to see what could persuade players to jump to the next game. Civ V is still at times more popular than Civ VI, and that could be the case for EU for a long time, especially given people are gonna wait to buy it given they're now familiar with the post-content strategy of Paradox Interactive.
Vicky III is one that would be able to improve on Vicky II (graphically, gameplay, accessibility), add new ideas to it but more importantly significantly widen their base if done right, and i'm unable to see how other iterations at this time could do that to a similar extent (and no, vicky II is not a bad game, it's just that it has much more potential than it has right now, and why it's the game i want to see announced most right now).
I think a Cold War game will happen eventually. I mean, East Vs West didn't fail based on the premise, but due to Quality Control problems.
Paradox was more than willing to put their stamp of approval on a Cold War game.
Also, I don't think a separate Modern Era game would be all that necessary. With enough design flexibility the mechanics of a Cold War game would work well in representing the Modern Era and if such a game doesn't extend to at least 2020 (it should, there's no reason why the Cold War couldn't have lasted longer, indeed, the end of WARPACT was a surprise to many policy makers in the West.
I also hope that launching nukes isn't just a genric "Game Over, you lose" moment. There should be severe repercussions for pressing the big red button (such repercussions would be diplomatic in nature, also dealing with the aftermath) in order to dissuade you from pressing it, but pressing it should transition the game from one about preventing Global Nuclear War to one about dealing with the aftermath of Global Nuclear War.
Modern era GSG have a few problems that I don't think could be overcome, and as no-one has been able to produce a qualitative game, i don't think it will happen.
1. It's gonna be way too controversial to depict certain countries. Some countries will be upset, banning the game in their country, and it's hard to be neutral as a game publisher. In historical games, it's less difficult, since what happened did happen.
2. The modern era isn't just that interesting, as there is often no "real goal" in the game, if you wanna go realistic. If you want the same intensity of war and things like that, it's gonna be over the top and not interest anyone, while if you go realistic, it might feel way too boring for a large part of the potential userbase, since warfare on a large scale is less common than it was centuries ago. Few countries have a standing army that large.
If they wanna take the risk, you're gonna need to overhaul the GSG system to make it interesting, and perhaps make it more of a political simulator focused on issues and elections, dealing with domestic problems as well as geopolitical conflicts or issues like climate change, nuclear proliferation or things like pandemic response, but less on actual warfare, except for some proxy conflicts, but conquer the world and paint your map in your nation's colour would just not be that interesting i think. They'll have to look more into how Democracy 4 and stuff like that works, than to their own games, because otherwise it's not going to work.
A fantasy IP could work (Total War also went with Warhammer for fantasy). It's possible to do. Alternate history is also an option if they run out of ideas, as Kaiserreich has shown that it could be popular if done right.