In response to the previous post, SD cards are a whole lot slower than SSDs. you might get 20MB/sec out of an SD card, while you will get 250MB/sec out of a good SSD. The SD card reader runs off the USB bus, so it is limited to 25-30MB/s because of that.
I would rather have an expresscard slot back. I don't need more than 2 USB ports because if I'm at my desk, I plug in a hub for just about everything, and plug in my EyeTV tuner directly since it is speed-critical. If I'm on the go, I never have more than 2 things plugged in at once. I might have my ipod, a hard drive, a flash drive, or an SD card reader (I have a multibody MBP with no built in SD slot).
The expresscard slot has a USB interface inside it, so a USB card could be used to add more USB ports. It is also the fastest connection for an external hard drive, since USB2 is terribly outdated, and even Firewire 800 can't keep up with newer disks.
Alternatively, I would be happy with a combo eSATA/USB port. It wouldn't increase the number of ports (Steve wouldn't want that), and it won't take up as much space as the expresscard slot, and it will let me hook up an eSATA drive without random kernel panics. Maybe if they move away from Nvidia chipsets there will be an extra SATA bus to make a combo eSATA port.
Of course, USB3, Light Peak, or Firewire 3000 would also be acceptable, but they are all still in development for the most part (and I think FW300 won't make it this time).
A dock would be great. They could make it magnetic! Also, I never use the CD drive, so they could replace that with ports, another hard drive, or a bigger battery.