No matter whether the child born to William and Kate is a boy or girl, apparently the new laws will give them right of succession above other siblings.
Apparently they've also changed the law to where the monarch or potential monarch can stay as a monarch and marry a Roman Catholic (maybe also Orthodox?) and their spouse isn't required to convert to Anglicanism, but non-Anglicans are still kept from the throne.
Unfortunately, the laws haven't been changed yet. Undoubtedly a large majority of people in the Queen's realms would desire these changes, but there's always an Elijah Harper
wanting his (or her, mustn't be sexist now
) fifteen minutes of fame.
BTW, even now there's no restriction on the religion of the spouse of the sovereign other than the one against Roman Catholics. (I believe the historical term is "Papist", so that would include all who are in communion with the Pope of Rome.) Several centuries ago when the current laws were enacted, nothing other than Protestantism and Catholicism existed in the minds of the legislators.
Now, I wonder what would happen if the young Prince "Arthur" wanted to marry the dashing Sir "Lancelot"