Well I don't know CK that well, but between EU2, Vicky and HoI2 the flags of the armies were very similar. You could even just copy a flag from EU2 to Vicky, and it would work fine. And from Vicky to HoI2, the shields were very similar, although indeed the flags couldn't change in HoI2

. Still the way they were displayed was very similar (not talking about the actual flags themselves of course, that would be ridiculous).
Another similarity I just thought of was, IIRC, that in CK the armies were also represented by a single man, just like in EU2.
Well the map in EU3
looks completely different than in HoI3, don't you think?
I was a bit confused as to exactly which parts of the game you want the mechanics to count and which parts not. Because you also said that, apart from "graphics", "warfare, economic system, state" are completely different between CK, EU2, Vic, HoI2; but when talking about the map, suddenly how it works doesn't count anymore.
All in all, IMO, HoI3 is lot more similar to HoI2 than it is to EU3; and EU3 is a lot more similar to EU2 than it is to HoI3. Both in appearance, and in gameplay.
Yes that would suck, but I'm sure they won't do that, the mockup Johan posted looked quite good.