Well, I guess it would be a safe bet to say that inspiration is not copy-paste thing
Whatever it was at the beginning evolved in time. I mean after so many years of Warhammer changing constantly, with new editions, new units, new characters it probably is impossible to narrow it down to a single nation for example. And even if it was like that at the very beginning, now we have the world that has a life on its own.
It is of course interesting in itself to find out what could be an inspiration, what was for particular game designer (and there were many who left their mark on warhammer) and what is for you in particular. Why not take an inspiration from your favorite historical (or fictional for that matter!) period and create a themed army?
As always topics have their own life too
I remember I started that more to learn about what inspires you in military history to try and emulate it in a very simplified form in your games. Now we are more about the culture of Elves and Ulthuan as nation. Feel free to continue!
But to add something new from my own I must admit that recently I am more inspired by Mongols and their art of war. The more I read the more impressed I am by their discipline, skill and precise coordination in battles. Despite the fact they were barbarians, their armies were not hordes of uncountable warriors who won battles due to sheer numbers.
I am inspired by their maneuverability and swarming tactics. I wish HE had reavers in core so I could try and create all cavalry MSU
Who knows, maybe I will be granted that wish with a new book!
Cheers!