Ancelotti has achieved just about everything that can be achieved as a manager. However, questions are raised regarding his manager's skills. This is because he has exclusively trained clubs with the most expensive squads, and although he has won trophies, he has left no lasting legacy. His leadership style and training regime have also been criticized.
From CS's perspective, Everton should be a perfect opportunity for him to answer the critics. To prove that he is not only a posh manager, but also a manager who can get dirt under his nails. A manager who can make the team better than the players' individual product. Through proper training, pedagogy, playing style, tactic, etc.
Personally, I believe neither Ancelotti has the skills nor the energy required, but this is precisely what Ancelotti must disprove if he is to be considered one of the greatest.