I really like the idea of a sack dress (or is it a shift dress? I just never know.). They seem so effortless, and breezy. My head if full of the Jamie & the Jones Staple Basics Midi Dress, the Elizabeth Suzanne Georgia Midi , and the Ace & Jig Harbour Dress.
I have tinkered with sack dresses before by modifying the Lou Box Top (original design by Sew DIY). And, I've made the slightly-more-structured-than-a-sack dress Lodo Dress from True Bias. There are plenty of pre-drafted options out there like the Tessuti Yuki Dress, the MIY Collection Fulwood Dress, the Mariella Walker Maya Dress, and the darling of sewcialists the Named Clothing Inari Tee Dress. I could have used any one of these patterns to similar effect, but propelled by sartorial hubris I decided to make my own.
I used the arm opening from the Lodo dress, an old school neck facing (shown below in Liberty Tana Lawn) bound at the edge to add a little intention to a dress that could easily be mistaken as thrown together, and the width of my Lou Box Dress with a midi length. There is no shaping, and a straight hem. Laid flat it would be a perfect rectangle. I'm pretty sure the fabric is rayon modal denim from Joann Fabrics. You've got to pay careful attention to the seam finishes to prevent it fraying into a pile of fuzz at your feet, but I think that little extra effort is worth the shimmery drape.