It does look like it's going to be a fun set, whatever it is!
Both Vivendiere and Russian Women Commies sound like great guesses to me. The Russian Revolution women in particular sound like they'd fit in alongside the upcoming WWI Russian soldiers.
And as simple a thing as the long, basic, utilitarian dress would seem to be - I suppose that it could even have been something that male soldiers might have worn at some surprising point in history around the world - those shoes combined with the dress do seem to be quite feminine, and would limit the scope to within a very curious choice of time periods over the last couple hundred years: they look rather Victorian to turn-of-the-century to me, in fact:

Not sure what these cheerful-looking gals are up to, but they happened to be one the first photos that turned up in my search results for "Victorian Dress", as an illustration for a conversation about corsets of all things!

Carrie Nation from the 1910s Women's Temperence activist movement that spearheaded US Prohibition of alcohol, with her famous Bible and beer-keg-and-booze-bottle smashing hatchet. I didn't see any version of the picture that shows her feet, but so many of my image search resuls kept reminding me of her.
Someone armed with some knowledge of the history of women's fashion coule, i bet, pin the style down to a very specific time and place, and it probably wouldn't take much to do so: very much earlier in the 1800s, and it seems that maybe dresses start sweeping the ground without showing off any ankle, while very late into the early 20th Century and the hem starts riding shockly high up over the ankle, to below the knees by the time WWI is very far underway.
With that in mind, I'm tempted to suggest that it's a Partisans set, but I don't think the style is quite right for WWII. The title of the first Partisans set does not seem to imply that the second would HAVE to be WWII French Resistance, though - "Partisans I - French Resistance" - the second set, it seems, could be for anywhere.
I'm thinking that any actual historical army seems unlikely, and this is more likely to be a "fantasy" set (in the broadest sense) than a historical one. Seems like some of the other previews recently have included coils of rope, tomahawks, and other possibly wild western bits - maybe a set of Wild West women is on the way? Wild West seems like the set most likely to me to have some commercial appeal.
As for that "not" Joe/COBRA near-future counter-terrorist skirmish project, I finally got around to looking at it a little more seriously, and I might have most of a plan for it now, with the short version being that the StarGrave rules set looks like it's almost perfect for what I have in mind (just ignore the minimalist default setting, and downplay/drop the surprisingly little amount of the book dedicated to psychic powers, alien artifacts, and spacecraft, beyond the minimal extent that they fit into the general pulpiness of the official Joe setting as I remember it from the '80s.) So, that's a pretty solidly playtested rule set, and now we're starting to see some perfect infantry figures - my plan just needs a little original alt-'80s/cyberpunk fluff, and maybe some rules on how to handle light vehicles and artillary as anything more than mission objectives.
I might take further thoughts on that to another discussion, though, as I develop it more....