Well July is already farming month, so I think we should introduce August as Air Month. We could have a magic elvish/fairy castle in the sky, but you have to do a super hard quest to get into it, because it is floating away and you need to kill the (level 226) creature destroying the things holding it down, and you need to fix the supports. (which are made of rune nails and something that will later also be used for carpentry)
Then we could introduce weather, and this idea:
http://runehq.com/RHQInn/index.php?showtopic=127255
Then, there would be a quest where you help Gnome Airlines establish new locations around RuneScape that you will then be able to fly too (but they will be set places, you just help build them).
My 2 cents (Which are worth 20)