I'm also a big believer in calisthenics and bodyweight progression, but it's worth noting that pistol squats and several other advanced bodyweight squats can strengthen your quads and glutes very well, but end up teaching you a different movement pattern from a biomechanically-sound (i.e. straight-spined) squat. It's basically impossible to avoid tucking the pelvis and rounding the spine when doing a pistol.

E.g.: https://www.youtube.com/watch?v=7NvOuty_Fnc

Edit: Shrimp squats are an exception, I suppose.