Not to be a broken record, but unless he dies before his mother, Charles will succeed the throne as King. There is no alternative legally.
Charles cannot legally simply abdicate. If he wishes to do so, he must convince the Parliaments of the UK, Australia, Canada, and all the other Commonwealth realms to first pass separate laws allowing him to - the same way his great uncle did. And those laws would simultaneously change the separate crown successions.
Given the republican sentiments in some of those realms, it would be very likely that one - or more - of those realms would take that opportunity to abandon the monarchy altogether. Or - one or more of the realms might not choose William but instead select someone else. More today than back in 1936, the result would quite possibly be the end of the personal union of the crowns - and possibly the end of the Commonwealth itself. Charles knows all this only too well.
And - at least as of today, when Charles succeeds the throne, Camilla will automatically, legally become Queen. British law doesn't recognize a morganic marriage. A King's wife is a Queen, no differently than the wife of a Duke is a Duchess, or the wife of a Mr. is a Mrs.
So, if people don't want that to happen, they had better start petitioning their PMs to get a bill passed that would create an alternative title - and it better pass before the present Queen dies, because trying to strip Camilla of the title once it happens would be very messy indeed.