Suppose there is an iWatch, it has a display, bluetooth and a lot of sensors...
What new features in iOS8 are useful for this theoretical device? This Yosemite and iOS 8 article basically spills it all.
iOS 8 and Yosemite allow your iCloud-linked devices to automatically find each other and connect. So buy an iWatch, sign in with iCloud once, and your devices start syncing data. How would you sign in on an iWatch without a keyboard? Well, it could work the same way the AppleTV can be setup over wifi: tap it on the iPad, a setup app appears and voila, your iWatch is configured.
The Apple TV now uses bluetooth for connection too and you don't need a Wifi network to find your Apple TV anymore. It's not that farfetched to imagine an iWatch that's bluetooth only that relies on your other connected devices (either Wifi or LTE) to get online.
Messages & Phone
The voice-messaging feature is an easy one. You get a message, see the notification on the iWatch display, hold a button, say your reply and send it as audio. No Siri or interpretation needed, you just send your words directly. I can imagine QuickType being useful here to. "Honey can you also buy some milk?" provides you "Yes, No" as possible answers.
SMS syncing is another feature. Having your sms's on the iPad is useful, but if you want to really dive into messaging, receiving those messages on your watch would be useful too.
Think it's a coincidence your Mac can answer iPhone calls? Well.. they can use that API for a watch too, wouldn't they?
Continuity & Handoff
You get a message, your iPhone sends a notification to your iWatch, you open it. Now you want to reply, but realize this reply will be a long one, and it kinda requires a keyboard.
Go to your iPhone, tab the Messages icon on your homescreen and voila, instant access to your message and start typing. Or vice-versa: you look up directions on your Mac, and your iWatch already shows the maps icon to give you your directions..
The recent contacts the App Changer could be a useful feature too. Nobody wants to scroll through a list of contacts on a watch. But flicking through your 5 recents contacts rendered in nice round photos? Sure, why not?
.. but directions need internet to get updated info. With instant hotspot in iOS 8 you don't need to fiddle with passwords, your iPhone/iPad/iWatch are automatically paired and online without any settings. It just works.
The obvious one. A watch with a pedometer, heartrate-monitor, motion tracker for sleep and so on. That Health app needs something to sync with right?
I could image an iWatch SDK 2.0 where Apps installed on your Mac or iOS device projects an extension to the iWatch. Foursquare with a check-in extension (if they still exist next year...), OmniFocus with a todo-list widget or Philips Hue providing a quick On/Off toggle.
But not immediately. iWatch 1.0 will be Apple only, just like the iPhone was.