Probably around 2000 or so, I bought a couple booklets online entitled "The 40-Yard Dash" and "The Vertical Jump" co-authored by Martin Rooney and Joe DeFranco. They were very solid pieces with very specific and useful advice for athletes and coaches looking to improve those particular metrics.
Although I liked those two booklets a great deal, I have not followed Martin Rooney's work much since. That has been an oversight on my part apparently because I recently read this article and it is outstanding!
Squat Like a Champion - Article by Martin Rooney
Presented in the article is this progression:
Low Mobility, Low Load
1. Body Squat, hands held straight out front, heels elevated
2. Body Squat, hands held straight out front, feet turned out
3. Body Squat, hands behind head, feet turned out
Low/Moderate Load, Average Mobility
4. Single Kettlebell Goblet Squat
5. Single Dumbell Goblet Squat
6. Dumbbell/Kettlebell Squat, held at side
Moderate/High Load, Average Mobility
7. Double Dumbbell Front Squat
8. Double Kettlebell Front Squat
9. Barbell Front Squat
10. Barbell Zercher Squat
11. Dumbbell/Kettlebell held-at-side Squat
12. Barbell Back Box Squat
High Load, High Mobility
13. Barbell Back Squat, heels elevated (shoe or plate)
14. Barbell Back Squat
15. Dumbell Overhead Squat
16. Barbell Overhead Squat
I really like the continuum presented here and think it's a great thing for people to refer to and try if they find maintaining good form on regular barbell back squats to be a bit challenging.
I might suggest the following modification - Most trainees are just not going to be using enough weight to justify calling their overhead squats "high load". Another category, "Low/Moderate Load, High Mobility", could be added as a parallel category to "High Load, High Mobility". In this category, squat variations like bodyweight squats w. arms overhead, overhead squats w. dowel/barbell/kettlebell(s), etc. would be included. It adds one more layer of progression to the mix and because the load is, generally, very light, would not be a large added strain to the system, and could be done concurrently with the "high load, high mobility" work.