As things stand at the moment and allowing for the Act of Succession, QE2 will be succeeded by Charles III (or George VII). QE2 will not abdicate as she knows the cost of that ré her uncle and father. She may create a Prince Regent for the day to day and William could assume that role.
One thing you have to bear in mind is that with all the cotton wool of the Palace and the age of both of his parents, Charles also has a fair number of years left in him - he could easily pass 90 and there is no history of dementia or debilitating diseases like that kicking around that could push him to surrendering his heritancy to his son before accession. There have been incidents of grandsons taking over, but usually through death of the Prince of Wales first - with the Georges 2 to 3 - Prince Frederick died unexpectedly in 1751, 12 years ahead of his old man.