Broken Rib fixes the problem

Yes, broken ribbing is the answer to the 'regular ribbing pulls in so much that the lace 'bludges' on the Horcrux Sox.

When you google 'broken rib' all the pattern are for 1x1 rib. And broken 1x1 rib is a lovely all over pattern with a vertical line that (while not identical) is attractive on both the front and the back. I like it for baby garments, since if you are careful with the finishing, they are reversible (good for the envitable mess). Garter stitch is baby friendly for the same reason, but Broken Rib has a vertical line, while Garter is strongly horizontal.

But ANY ribbing can be broken. Rib patterns other than 1x1 loose the 'reversible' factor, though. They have a definite 'front' and 'back'. The technique is dead easy, too.

Round 1 (all odd rounds): Ribbing pattern (in the case of the Horcrux Sox - K3, P2)
Round 2 (all even rounds): Knit

in flat knitting, Row 1 (RS) Knit, Row 2 (WS) Ribbing pattern reversed
If I were knitting the ribbing from the Horcrux Sox flat, Row 2 would be P3, K2.

Simple and effective. The "down" part of the rib, which is in reverse stocking stitch when knitting a true ribbing, changes to garter stitch in a Broken Rib. Therefore it is utterly flat. It will still pull in just a bit from the columns of stocking stitch, and you keep the visual columns of the ribbing.

I am happy with my Slytherin socks so far. Next decision point - the pattern's designer has a stocking stitch heel and I really prefer a heel with heel stitch or eye of partridge. Heel stitch would continue the column theme from the ribbing, but eye of partridge would give another texture - and three textures may be too many. Maybe a version of Charlene Schurch's Heel Stitch with Garter Border?