I had a Perfect Cardigan a few years ago; it was a V-neck, ribbed around the waist so it followed the line of any dress or top-skirt combo, but was looser at the bust and the sleeves, then ribbed again at the cuff. It sat perfectly on the waist so it didn't "interrupt" the skirt, looked lovely straight-on and in profile. It just worked. I lost it ages ago and I still think about it because I HAVE LOOKED EVERYWHERE for a similarly flattering cardigan.
Every cardigan in the entire world seems to either puff out at the waist and then in again at the hip or hang straight down (which is a bit better but not ideal).
The occasionally successful option for following the line of a dress seems to be a ribbed V-neck, as you can just do up one button and the rest clings around your waist and hips, but they tend to be very tight at the sleeve which is not very comfortable if the dress has sleeves, and often are too long to work with many skirts. Also even they are often shapeless.
The closest I can find it those vintage-style cropped cardis, but they're almost always short-sleeved which baffles me, and almost never V-neck meaning you either have to do it up to the neck or leave it entirely open, they just don't sit right otherwise. I also don't want something super cropped, just something that allows me to wear a skirt without looking like I'm wearing a nappy in profile.
It's so annoying! If I'm wearing a lovely fitted dress that nips in at the waist I don't want to cover it in a big bubble!