Well...I don't know who they will marry (because who does, really!)
BUT I think they will have their share of fame much like the Queen's grandkids. The press eats up the going ons in William, Harry, Peter, and Zara's private lives because they are young and good looking. Basically, they make good press fodder!
If Frederik doesn't have children any time soon, I can definitely see these boys emerging in the spotlight. Lets say Fred doesn't have kids for five more years, Nikolai will be eight years older than his cousin, the heir to the Danish throne.
While his cousin will be "more important" to the country, Nikolai will be older. When he is 18, his cousin will only be 10!

So, I can defintely see people wondering who they are dating and what they will study, etc, etc.
I wonder if the boys will marry into royalty? Their mother is a "commoner", so I doubt they worry too much about the royalty issue. Also, I wonder if the boys will be more free to marry people from different backgrounds and races, considering their mother grew up in Hong Kong?
-Mirth