[Gorwingel]

I could never be satisfied

I personally really would not want new books, because especially LOTR is quite complete and is a masterpiece, but as others have said additional details about events and characters in the current books would be nice

Of course there is that ever present "Do Balrogs have Wings?" question which we will never truely know the answer too
It would be lovely to have a definite answer on that topic, so we could finally put that debate to rest (which will never happen)