HOI4 mods probably would benefit from an EUIV styled peace-system. But a lot of mods benefit from not having to perfectly simulate everything- for instance Equestria at War is spinning most of its stuff from whole cloth, they likely wouldn't want to emulate population statistics. Same for Old World Blues and the like.I'm wan't being literal, but I wouldn't dismiss the possibility of mods taking a sci-fi direction. CK2 had that somewhat popular space mod, despite the engine being much less suitable for that kind of setting. I think the combination of playing as non-state actors plus population simulation mechanics will be too tempting for modders to pass up.
I agree that HOI4 has its own strengths as a grand strategy game with wargame elements, offering plenty of opportunities in settings with total wars using modern weaponry. That said, I think this also makes it somewhat less flexible than Victoria 3.
I do think we'll see more fantasy mods though for Project Caesar, as the introduction of characters and micro-nations lends itself very well to them- alongside the ability to have unique cultural units. And like I said, the Great Works of Art could be retooled into a full Artifacts system, if Paradox themselves don't go that route (I honestly think they should). I think the one thing they are missing (outside of a full on fantasy race mechanic) would be some sort of 'expeditions' mechanic that I think Paradox should consider adding (probably in a DLC).
On the topic of other fantasy settings to adapt, I'd love to see Dishonored's setting, but the games only give modders so much to fill in, since they are more action games first and rpg's second. I just love the idea of a small set of civilized islands with one ginormously huge continent to explore, so maybe someone will come up with something more original.
On a similar note- I wonder if Project Caesar will allow for full on steampunk settings. Anbennar has some of that to an extent, and EUIV had the early industrial revolution. I haven't personally touched Victoria, so I don't know how that game handles the Industrial Revolution, but I've been told it's mostly an 'economics simulator' than anything else.
- 5