It's an intellectual exercise, revolving around how aircraft could have been made better with technology available at the time.
I would say that other developments like gun/cannon and missile design would also be discussable to a point.
- The existing specification: Basically, the idea would be working within the existing specification, but you could modify or change anything within the boundary of it.
- A different winner: Sometimes the problem wasn't the design so much as the winner to the contender
- A more realistic/practical specification: Basically the specifications are made more realistic to allow a practical design to be developed.
I would say that other developments like gun/cannon and missile design would also be discussable to a point.