Out of all the factions in "western europe" the one who would fit most is prussia (unless their already here and i havent seen them)
as pointed out on the same page you posted your suggestion
, Prussia was a small poor duchy under Polish control, their inclusion in the game wouldn't make much sense.
I have to agree with Hinkel and Viccard, Prussia wasn't that significant back in XVII century, it was small, polish fief. However, HRE would be a very interesting option, same about Austria. Mod lacks armies with infantry.
The Hre is not so suited for its depiction as a single faction, it was composed of small mostly independant states. they had rivalries and alliances and wars with each other, a unified Holy roman Empire faction would not make sense