I am pretty happy with iOS but wouldn't mind a the ability to remove some of the stock applications, Stocks, Newsstand, Memos and Compass having never been used on my iPhone. Even better would be not having them as standard and allow users who require such applications to download them from the App Store.
I am not a huge fan of the music Application. The icon is ugly and I would like to have podcasts be part of that application once again. I also dislike the separate videos app. So I basically want it to go back to an iPod app.
skeuomorphism is the big bad at the minute and naturally I never want to see faux stitched leather again! Brushed aluminium has never bothered me though.
One of the posts above mentioned having the multitasking tray be activated by an upward swipe gesture similar to the downward swipe of notification centre and while I like the idea it would most likely intrude on a helluva lot of games, paper toss for instance.
Game Center, if ever there was an iOS app in need of a redesign, this is it. Everything about GC is horrible from the cheesy casino-a-like design to adding friends. I t would be nice if it scanned your contacts and automatically added them to the app.
Maps, I barely use the application but just trying to input addresses into my contacts was a chore, often with the maps app taking me to the wrong place. I don't really see the point in flyover either.