As sdm said, a dropdown bar in the homescreen to toggle on/off connectivity features, i.e. 3G, Bluetooth, WiFi. This is extremely important to me, as I need Bluetooth for my car, but NEVER use it for any other situation, and it's WAY too much of a drag to dig to turn it on and off before and after each drive.
That aside, I will fill you guys in on my theory for the "Next Big Thing" after multitasking.
My theory is that in update 5.0, or maybe even with iPhone HD as a "one more thing" (although I doubt that), I think we will see a widgets page in between the 1st Apps page and the search page. I don't know about you guys, but with the features of 4.0, the only REALLY big thing that I feel I'm missing is one centralized place to see all the basic information I need to see at a glance.
In my opinion, Apple should offer some as stock widgets which you can turn on and off, connecting to apps such as weather, stocks, etc..
In regards to third party programs, I feel that there should be two options... The ability to write a standalone widget, and the ability to include a widget in an app. And example for the former would be Macrumors Headlines (standalone widget), and an example of the latter being Facebook Notifications & Shortcuts (which can be turned on or off, but comes with the regular Facebook Application).
That's just my idea for a Widgets system, which will probably be very far from Apples implementation. However the idea of the system itself is something I DEFINITELY think will be the next big software update feature.