I recently came across this list of people in the line of succession to the British throne. People have taken the trouble to list the first 800 or so people in line, taking care (according to the Act of Settlement of 1701) to skip over anyone who is Catholic, is married to a Catholic, or was at one point married to a Catholic. (Query, for anyone who knows: This is weird.... I assume that if you renounce Catholicism you're back in the line of succession? What if you marry a Catholic but your spouse then renounces Catholicism? Are you back in line, or is removal from the line of succession permanent? Would someone who had divorced a Catholic have to get their ex-spouse to renounce Catholicism to get back in line?)
Point 1 is that what would rock is if Prince William started dating a Catholic. Constitutional crisis! (The Catholic prohibition is sitting there, but it's apparently never been used, and as matters stand today, the prohibition won't be used anytime soon, since the first disqualified person doesn't show up in the list until the 20s.) Of course these British constitutional statutes are just statutes, but (1) it's an important statute, (2) under the Statute of Westminster of 1931, this may require the assent of all the Dominions, and (3) they'd probably have to do some rethinking of this whole King-as-head-of-the-Church-of-England idea.
Point 2 is that the King of Norway is 60th in line of succession. Now 60th is pretty far down, but Norway has been trying to take over England since the year 787! Is there a political thriller plot in there somewhere: The Lindisfarne Legacy? (Note that apparently in V for Vendetta, there's a reference to a "Queen Zara," apparently implying that Zara Phillips, who's now 11th in line, was the most senior Family member to survive a 1980s nuclear war. For Norway-like plots involving currently non-functioning thrones, the heir to the Romanian throne is at #82, the heir to the Serbian throne is at #89, and one pretender to the Russian throne is at #107.)
UPDATE: I know what you're thinking: Isn't "The Lindisfarne Legacy" a course at St. John's College in Durham, England? Not the same thing. You're also thinking: Didn't the Vikings already rule over England from 1016 to 1042, during the reigns of Cnut and Harthacnut? It's tricky to draw these sorts of distinctions, but Cnut is generally thought of as a Dane, not a Norwegian. The Queen of Denmark doesn't come into the British succession until #209.