My suggestion to Turbine, keep an eye on your competition.  Specially those with wings.

want to see nazguls and giant eagles in game?

i think its a lore based game, and its quite odd to see every creep flying on the fell beast.

If they ever upgrade the graphics engine, flying can be added and at the same time not really changing anything. Right now we have instant travel but with no definition how it works. Define instant as a Giant eagle being summoned to ferry you off to said destinations. You can turn the flight on or off depending on how many times you have already seen it and what mood you are in.