I've been thinking of a way to get something similar to a below-the-knee cagoule with plenty of ventilation and good protection for a pack and when you sit down (so that you can cover your legs) and yet not be as billowy as a poncho. However I look at it I always come back to clothing from the Middle Ages and Renaissance. They really knew how to make protective clothing.
I was thinking to use something like an ID Silcoat Cape and instead of rainpants, use a waterproof apron or "tabard" underneath. I'd slit the front of the cape, put in a zipper, and allow it to be pulled back when not raining hard to open up the front of the cape.
Some ideas come from capes, tabards, mandelions, cowls, and hoods:
(The top of the sleeve of the tunic under the hood has an "archer's notch sewn in so that when you raise your arm the tunic falls away and doesn't restrict your motion.)
I was thinking to also use Bill Fornshell's idea of the insulated tunic/quilt and also the Fin Finbar Hood, but a better version of the hood, using a Montane Extreme Smock Hood fitted with a shoulder skirt that I could use when sleeping in the quilt.
Here are some more ideas for Medieval and Renaissance clothing. It may be funny in many ways to look at this stuff, and a lot of it wouldn't be practical in wind or climbing steep trails, but if you stop and think about what people back then needed to stay warm and dry in the non-mechanized world they lived in, their clothing can give us a lot of hints for our own way to design for the outdoors.
What do you think?