I don't think that Albert will ever marry. I think he feels the same as many of the Monegesque people do, that Caroline will make a wonderful reigning Princess. I think he may stay with Charlene (or whoever), but never make it official so that his sister can be sovereign, and then her children. Andrea, Charlotte, and Pierre seem to have been raised to be the first family of Monaco.
I think that Pierre will be first, simply as a mistake. He seems a romantic sort, and may rush into it when he think he's found the perfect girl, but she may end up disappointing him.
Just my opinion.