EU V has likely been in the works for a while, like others have said, though I feel like with each DLC coming out it might increase the urgency for the sequel. Leviathan was a disaster and much of this next update is fixing the mistakes of the previous one while the actual DLC is just missions, skins, events and music probably. The only actual mechanic so far seems to be the different branches of missions but that isn't a gameplay mechanic, really more flavour or effect changes. And quite frankly the content they have made and have had ideas for is highly questionable, doom cities, bankrupting a continent, American empires, etc, and while they might tweak some things they also remain quiet on other fronts and I'm sure they'll come up with another crazy idea in the next DLC. On top of that we know the update will not include map changes of any sorts so there goes a large part of the update. Quite frankly, I only see backlash growing from this point forth and it's just going to speed it up with each new update and DLC. People want provinces, developers don't, people want tags, developers will add releaseables, people want logical mechanics, developers release crazy ideas or nothing. The roadmap for EU IV is pretty shaky right now. I'd love to be proven wrong though.
Regardless of all that though, I do seriously hope for EU V soon. The series needs a shakeup and to the core, not another layer of paint on a broken wall of a crumbling house. Even the devs have repeatedly stated they can't do things because of old coding, even just tweaking starting Portugal breaks the whole game apparently. I jumped on the EU V train during the Golden Century DDs, I can't even get mad anymore, just disappointed.