I would think that Spain is holding on by just a bit, but would be "next" to go - followed by Belgium. And interestingly, they have two very different heirs, the Prince of Austurias and his "flashy" princess (I am not saying that she is! but the perception....) and "dull, plodding" Philippe & Mathilde.
The interaction between the history of a country, the history of the particular royal family, the world political and social climate, the perception about the heirs to any throne, and one's personal background is interesting at the very least.
When it comes to the UK - I personally think it should just stop after Elizabeth. A monarch like her comes along once in a millenium.