The Tyros 3 will have more than a half-gigabyte of sound ROM (maybe compressed to something smaller). Included among the new sounds will be the biggest piano sample ever in an arranger, probably over 100MB. I think these extrapolations are pretty safe.

An interesting new feature would be to host software plug-ins, maybe VSTs or maybe Yamaha proprietary software plugins. But I would think this would show up on the workstations first. Another possibility is the appearance of physical modeling of some instruments components, such as piano soundboards.