Looking around you can find different comments from interviews and articles which indicate both new features and new products as well as stability in established products.
From an article in makezine we get specific mentions of USB3.0:
The future of Raspbian
I think a lot of people forget the Foundation’s educational mission, and the importance it places on backwards compatibility with the 8 million Raspberry Pi’s already out there. When you’re dealing with computers in a classroom, not making all your teaching materials out of date overnight is vital.
The future holds new boards
“I’d like to see USB 3.0 added, as it really is the universal solution for adding peripherals — especially higher bandwidth ones like disk drives, network interfaces — and removes the requirements for things like SATA”
— James Adams, Director of Hardware Engineering, Raspberry Pi
But while I think any hypothetical Raspberry Pi 4 may well have USB 3.0 at this point, I also think the new Raspberry Pi 3 will almost certainly have a longer life than the Raspberry Pi 2. Perhaps two, or maybe even three, years rather than the one that the Raspberry Pi 2 survived. So don’t hold your breath waiting.
As for the frequency of major hardware updates we can look at TechRepublic:
Can we expect a new Raspberry Pi each year from now on?
Upton is adamant that the foundation is not about to start bringing out new Raspberry Pi boards each year. The rapid release Pi 3 is something of a one-off, he said, made possible by a combination of technical and cost factors. The earlier work the foundation completed on designing the Pi 2 paved the way to begin using the more powerful chipset found in the Pi 3. This technical readiness coincided with a reduction in the cost of producing the board, which allowed wi-fi and Bluetooth support to be added without increasing the price.
"We're kind of at the end of that particular roadmap. I would expect a longer pause, a couple of years at least, before any kind of major bump to the platform," he said.