Ronaldinho gaucho would make a great series just like the tv that is also very good his series tells the whole story of him since he was a child until he first arrived at Boca Junio!
Deion Sanders might be a fun one to watch with all of his antics on and off the field and with him being a multi sport athlete would probably be one people would want to watch. They did one with Bo Jackson already but doing one with Brady a few years after his retirement could be pretty good too. You could do one with the Cowboys of the late 80's to mid 90's too if they could do some research on some of the stuff that was covered up for them during that time.
I’d personally love a Wilt Chamberlain. Similar to the Bo Jackson documentary it could dive into which claims about his insane athleticism were true. Could he really bench 500 pounds with and 48 inch vertical and a track stars speed? If so he’s probably the greatest athlete to ever walk the earth given he’s 7ft tall. There’s videos of him blocking shots at the top of the backboard which I’d love if it was verified true. His life off the court would be quite entertaining as well.