In a similar vein to Allistair's posting regard static methods, I'd love to see Interfaces in Jade extended to include properties. This would then logically extend to being able to define MKDs of interfaces and use those MKDs as automatically maintained inverses. This would make an already powerful, but probably underutilised, feature of Jade even more useful and may increase the usage by developers as they start to see more of the benefits of using Interfaces.
Cheers,
BeeJay.