Slitherine

A river terrain that is not generally fordable, but can a engineer unit can build a pontoon bridge over it.
This would come in handy in a number of battles both on the Eastern and Western fronts.
You could use a dark blue colour to separate it from the current river.

Forest/jungle decorations that will cover a whole hex, not just a single hex-side.
Useful for adding some life to difficult/mountain terrain. In the Pacific these terrains were often covered with jungle.

That is exactly what I'm looking for right now. In the absence of a new terrain type for large rivers, do you know of any way I can make a river unfordable except by building a bridge?

Not much river options there besides using water tiles like lakes or placing impassable area borders. Bridges only work on true river tiles.

What’s also missing are two city-house deco options: upper-left and bottom.

I'd settle for something like this: being able to make a river unfordable by wheeled vehicles, but okay for infantry to traverse without a bridge.

You could change the river movement cost/possibilty for all kind of movement modes in Panzer Corps, but this is unfortunately not possible in OOB. The only river settings I have found are in climates.txt with river penalty.