I find it amazing the PDF is $60

thank you none the less
If you're particularly curious about
Draw Steel mechanics, you could look at the
Steel Compendium site (
Rules Index - Steel Compendium: Draw Steel Rules ). The
Heroes book is under a rather generous license that permits the use of much of its text, although neither art nor layout.
I'd agree with the D&D 5E / PF2e / CoC 7E being the largest ones, with D&D 5E definitely the 800-pound gorilla in terms of market. Regarding PF2e, it
might be interesting to create sheets that are intended for certain variant rules (notably, Free Archetype and Ancestry Paragon, because these increase the number of feats that characters take; and Dual-Class, which complicates things further).
If you want something slightly different.... well, you could always look into designing sheets for ships and crews of
Traveller. There's probably space for an autocalc sheet that tracks the numerous properties of Traveller ships, heh; or even an autocalc sheet that would compute planetary stats and properties, because that system (at least in Classic Traveller) is a bit crazy and involves a decent amount of math.