I do think that Charles is going to end up as something of an interim monarch. A bit like when companies bring in a new boss to oversee a restructure and change. Charles is the one who is going to have to make some big and lasting decisions covering everything from who gets royal titles, who gets security etc right through to how the monarchy deals with its past, the commonwealth and losing realms.
It's sad in a way, he has waited so long for this and I don't envy some of the decisions he will have to make. I think he'll be remembered much more for being the Prince of Wales and all that he achieved when he was, as opposed to being one of our greatest Kings. William on the other hand I don't believe will have that problem, by the time people get used to him being the Prince of Wales (I for one still picture Charles!) he'll be about to become King anyway probably.
There's some big decisions to make in the coming years and I hope Charles is able to almost take one for the team and actually make them, even if it makes him unpopular in some circles, but ensures everything is tied up and ready for William in a way that Queen Elizabeth didn't do for him.
Charles to me is almost the middle man, guiding the monarchy from the reign of his mother to the reign of his son, ensuring the transition is smooth as we go from two drastically different periods of time and societies. I feel like these are the transition years from the past and Queen Elizabeth to the future and King William