Since that is a real key, then, I think it must open something literaly. But what?
In my yume mind, I want to believe that is the key to their new house.
In chapter 72 we find out the mansion will dissapear because of volcano eruption (I think is kuran mansion because of the rose yuuki left behind). So, the kuran couple must live somewhere in the future. Since they want to start over, a new house is a good start.
But what do you think that key purpose is: to open a closet where is the solution of all problems?
ps: if this kind of thread exist already (i did not checked before) the mods / admin can delete it or move it somewhere else.