The biggest improvement to usability for the target educational market would be a 2nd SD card slot. On the old computers that had only one floppy drive it was a pain in the butt to copy a disk. The source and destination disks would have to be shuffled several times as a block the size of free memory was copied each time. Having a separate boot / basic software card and a 2nd card slot that can hold various data, whether a classmate's shared card, a big application, or other things would also make the machine far more usable.
An accessory that would make the system far more practical for kids would be a carrying case that can accommodate a low-weight, low-profile power supply, cord storage, protected computer storage (it's going to end up on the bottom of a 10kg bookbag and dropped from three feet on a frequent basis) and if possible, a low-profile keyboard and mouse storage. (Recommended standard human I/O devices should be offered at a good price with the RP.) There should be spare room for other items such as expansion cards, a documentation booklet, and secure storage for SD cards, too. The target size should be about the same as a mid-size textbook.
Another thing that could be crucial in the adoption of RP in education would be to copy the factor that made Arduino so popular - a standard expansion interface with add-on cards available, both from the originators and from 3rd parties. Different people want to do all sorts of things with the computer, but it would quickly become far too large and expensive if they were all built in. A standard mechanical, electrical and software interface is needed, preferably using the existing ports. The first such add on card should be a port expander that gives physical connectors for all the USB ports that the internal hub can handle. Other early expansion cards or shields could include:
an alternative I/O module for embedded uses,
a breadboard module with some digital I/O, A/D and D/A
a rechargeable battery and power supply module for embedded or tablet uses,
a sensor module with microphone, 2 or 3 axis accelerometer, user-hackable header inputs, etc. and
a motor-control module for robotics.
These should be designed so that multiple cards/shields can be used together. Perhaps a bridge from one of the RP interfaces to the Arduino interface could speed up adoption and instantly allow many existing shield products to be used, at least if there were an appropriate bit of software available for emulation and signal translation.
A plastic case for the RP itself is needed. An optional case that can accommodate expansion cards would also be great. For instance I can envision a flat case about10 x 15cm or a bit larger that can fit the RP and 3 cards all in the same plane, with enough thickness to allow a 1cm battery module on the bottom and an 1 cm LCD on the top, with the card layer in between - about 4 cm thick altogether.
The price needs to go up by 20% for qty.1 purchases, with a discount for distributors. This will actually lower the end-user price for those living outside the UK, and make the boards far more available worldwide.