Ancelotti never won a world cup. It is the chance to win a world cup and complete his career with the most prestigious trophy in football. However, he is already one of the most successful coach in history even if he may fail to win a world cup title with Brazil.
This will be the final target of his coaching career and if he succeeds in presenting the title for Brazil then he will be one of the most successful coaches. But even so, it is not easy to present a fairly prestigious title because in recent years Brazil has lost its best form so Ancelloti must make a difference from the previous coach.
Since Ancelotti never succeed with a national team, I'm not really sure that Ancelotti can instantly improve the performance of Brazilian team. But I believe Ancelotti has the ability to change Brazilian team, he has proven his ability by winning varied titles. Ancelotti is also very familiar with many Brazilian players. So, it will make it a bit easier to rebuild the team.
To improve the performance of the Brazilian team will not be difficult for Ancelotti because Brazil has a number of good talents in the field of football, but the target to immediately present the championship title may be difficult because of the tight competition between several countries that are developing rapidly in their football so that Ancelotti will certainly get a much tougher challenge than coaching a number of clubs that he has handled.