Plan:
1. Copy existing jacket pattern to tissue paper. Will use tracing paper...
2. Build a jacket using some scrap stretchy fabric of some sort (not the fancy stuff, not exact stretch, but hopefully close), minus zipper!
3. Check fit.
4. Adjust fit, deconstruct adjusted jacket.
5. Transfer adjusted jacket to pattern
6. Build jacket with new fancy softshell.
Getting the collar length is going to be a bit of a challenge.
And I think the bottom of the jacket probably should be a straight line... I think I flipped the wedge part. I will have to watch that.... I guess I can cut the jacket slightly long and adjust the bottom, however, as it needs to fold back over on itself.
Someone's video on shortening a separating zipper.

@ai6yr open-lower-back wedge cut, tied with strings.
Go kink/medieval on that bitch.
@ai6yr DOOOOOO EEEEEEEEEEET. You WANT IT.
Search your feelings. You know it to be true.
@ai6yr BTW one of the weirdest but comfiest pieces of clothing I own is an odd combo/mix of rain shell and a short cloak. It has wide sleeves, a hood, and closes by buttons front of shoulder/collar bone, one of those turned inside (told you, weird)
It looks like something out of a new Star Wars movie set. I'd wear it more often, but I don't want to look TOO MUCH like a scifi movie extra, walking around.
@ai6yr make the collar a reversible hood.
The fastening in front must be loop n peg. Bonus for the teeth or claws of whatever tried to kill you last.
Yeah, it was a very odd response. Internet ppls can be weird.