@bibbigo @jellybug78
For legs I have some ideas but I'm not sure about the exact level since not many train them, I'm thinking of making a post with progressions for all the hard leg exercises and an estimated level on the OG2 charts.
For Nordic curls after the normal version the next step is with arms overhead (still intermediate) and 90° hips one leg curls are around the same level as normal ones so they can be a branching path or the continuation of the progression.
I think one leg Nordic curls are advanced or elite, only 3 people have done them, and only one of them close to perfect form, but maybe if more people trained them they'd reach them, since I guess elite athletes in speed and explosiveness sports would be able to do them.
Natural one leg press/friction resisted pistols could be a progression after pistol squats, since on a smooth surface one rep felt like a pistol with 40% BW for me, so maybe branch pistols into weighted pistols and one leg press progression (partial -> full ROM on smooth surface -> full on less smooth surface - the latter would be advanced, and on a very rough surface even elite). Unweighted pistols are late beginner, not even intermediate.
And bodyweight leg extensions are another option for legs, with advanced or elite one leg long term possibilities, but there are several practical variations (sissy squat, kneeling leg extension, Matrix squat) and many possibilities for progressing them, I'm not sure which ones to suggest unless you want to put multiple paths in the chart.
Addition: since another thread regarding progression charts has just appeared, I brainstormed some ideas there in
this comment.