Allon said:definitely do leg extensions afterwards.
I also do calves and hams afterwards... all leg muscles, after squats.
I'd move on to hams and calves, too, but I wouldn't do any other quad work after the 20 reppers. Done properly, breathing squats probably stimulate more growth in one workout than leg extensions could in a month.
Anything beyond that would be redundant--a drain on your body's reserves IMO.