Mills has to clear the fitness test as he suffered a quadricep strain which ruled him out of the T20 Blast in England.
De Villers is likely to play in the upcoming season with most of the teams in the race to sign him.
Johnson will be turning 37 as the Australian summer begins.
The eighth season will kick off on December 19.
Root is presently playing England's three-match ODI series against India.
McCullum plays with Lynn as his partner in the BBL and Williamson was companion in New Zealand.
Watson's performance in the PSL and IPL has given his T20 career a new lease of life.
Boyce has also donned the national colours in 7 contests and has 8 wickets.
Nair's action was "markedly different" by the officials.
Big Bash League clubs are lining up at AB de Villiers' door for his signatures.