Since I'm interested in historical costumes as well I tend to find inspiration from history, and the TLJ Leia coat has screamed mid-16th Century loose gown for me.
Margareta Eriksdotter Leijonhufvud (1516-1551)
|Margareta Eriksdotter (Vasa), dead 1536 Copy after: Mäster Hillebrand|
Then I found a photo of this extant dress
|garment of Margaretha Francizka Lobkowicz née Dietrichstein, 1st half of the 17th centuryRegional Museum, Mikulov|
With inspiration from this time period I opened up my "Creating historical clothes" and used their pattern for a 16th century loose gown as the basis for my mock-up.
I changed the neckline in the front though to continue as a straight edge from the center front, this gives the collar a large lapel.
I trimmed the collar down and curved it to match the references.
There was a crease going from the shoulder seam and all the way to the front though, so I added a dart that continued up from the shoulder seam.
I was very happy when I found this reference photo that shows that there is a continues seam, from the sleeve, over the shoulder and up all the way to the collar.
I still want to tweak the collar a bit, but then it's on to the sleeves.
One thing that is guesswork at this time is the back. I don't know if there are any seams there at all. At this moment I'm going to treat the back as one large piece, if we get a reference for the back it's easier to add the seams in later, than if I make seams and it turns out that they aren't there.