This got lost in the other thread question about historical ones, but at least 1 of the 2 could be wild enough to deserve its own discussion anyway, and maybe both.
Suppose the ship carrying the Portugese royal family had sunk on its way to Brazil? Not as likely because the French fleet was already in Davy Jones' locker, but a storm might hve done so if they left at a different time. Who would hve been Portugese monarch then?
Also, Spain - looking at their monarchs, I'm struck by the fact that if Charles IV happens to get killed by a mob and then Ferdinand VII dies in an accident during french occupation of Spain, you'd have to go back to the other children of Charles III. (Side note - I never realized the enormous number of names some of them had!

Only remember being amazed Prince William of Brtain had 4 when he was born - William Arthur Phillip Louis IIRC; stunned at someone having 8-10) Quite a few died without issue befoer this, though a few survived. I'm not sure if it goes to the next oldest male's heirs
http://en.wikipedia.org/wiki/Ferdinand_I_of_the_Two_Sicilies or to the heirs of the oldest
http://en.wikipedia.org/wiki/Maria_Luisa_of_Spain (who didn't have a lot of names but made up for it with tons of kids

) but that could be an interesting situation to see who succeeds after Joseph is deposed and order restored. (I hope I got the two right in skiming through all those names.)
Either way, it probably creates interesting butterflies elsewhere, too.