Some reasonable guesses:
- Better Display (not quite “Retina” but close)
- Faster processor (A6?)
- More RAM
- Siri Integration
- Better camera
Some “Out there” (and semi-informed) guesses:
- Thunderbolt connectivity for faster transfer AND charging (goodbye, iPod connector)
- NFC
- Wider home button that supports gestures
- Apple will produce and sell their own Bluetooth keyboard case $79
- Wireless syncing between iOs devices (game status, files, you name it – start on one device and end on another)
- You can use the iPad 3 as a sketchpad for your mac (like a Wacom tablet) with a new drawing app from Apple (the return of Macpaint?)
- A projector attachment (for iPhone 4S or iPad) $99
- Passive 3D mode (a la Nintendo 3DS)
- New app called “Channels” that lets you “record” and stream TV shows for viewing on your iPad