For instance:

• Ballistic flight -- going up and down in the same spot is one thing, but launching up into space with a side velocity and coming back down 5,000km away an hour or two later would be far more interesting. I would come up with a prize for distance, or distance within a certain time window (doing it in more than six hours is not nearly as interesting as doing it in under 90 minutes).

  

• Larger payloads -- while launching three people means you can take passengers, hitting the same altitude with say a pilot + 1000kg means potentially you could take serious sized payloads to that altitude (like complete launch packages for micro satellites). This is not just a matter of a large rocket on SS1, it would require stronger wings, a probably a different command capsule configuration, and a significantly bigger carrier plane (has to deal with all the additional weight, which will also mean bigger engines and more fuel, which is also more weight and stress, which means even bigger wings, which is more weight ...)

  

• Hybrid engines -- to make this even more efficient, and to pave the way for orbital flight, we've got to rely even more on jets, or more specifically SCRAM jets. While Nasa has flown an unmanned drone successfully, we really need a manned flight that combines SCRAM and rocket flight, getting us to higher altitudes and higher velocities with less fuel (and much less oxidizer).