I think after the blocksize debate is over and after there is a good scaling algorithm integrated I see for Bitcoin following use cases with potentially exponential growth:
1) Digital cash for ever day use
2) Underlying protocol for any kind of trustless verification
3) Store of purchasing power
Everyday usage as a digital cash will need that Bitcoin gets used as form of payment for works very frequently, will need that merchants are accepting it all over the world, and will need improved privacy. Then we will hit worldwide records of usage.