Long skirts were very in during this period, though the length came up towards the end of season 2. Love a great skirt from the show? Shorten the hem to more modern length for an instantly wearable look. Keep the interesting details like the buttons on the back of Mary's skirt or the matching belt on Isobel's. For a more flowy skirt, try a half circle skirt. For a slimmer look, go with an a-line.
Sometimes you'll get lucky and find a modern pattern with similar lines.
Vogue 8648 would be a great starting point for Mary's garden party dress! I'd start with the yellow version and add details. Cut the pieces so the stripes line up with Mary's (or somehow interesting), cut the waist pieces in a darker solid for a belt look and add cuffs to the sleeves. Lace trim around the neck for that finishing touch.
Fallen in love with a great jacket? There are a couple of options here. A lot of jackets from the show would look great with jeans! For a more modern look focus in one one detail-pockets, a unique closure, or a great color and apply it to a modern cut jacket.
Vogue 8333 is a great place to start. It has classic lines but the little side detail makes it special. Change the collar, closures or color.
Vouge 8621 is a much more modern pattern but still captures that spirit of specialness all of the Edwardian jackets from this show have. This is a great option to capture a color that's been calling your name.
Don't forget to think outside of the box. Wouldn't Mary's riding jacket make a great coat dress?
I feel like a lot of the blouses would be very wearable with jeans or a modern skirt.
I've got many more ideas, but this is getting long so I'm thinking of doing a series of posts on this topic. So please post questions, requests and ideas!