There's always improvements to be made to our propulsion systems. Here's a list of ideas that could be used to pick a project for a future year. Please add to it as you have ideas!

Propellant Rework

Look at the Ocean Water discussion section. Other changes could include adding compounds to improve ISP, such as (link to TRF thread).

Composite Case

This would be a long term project, but the mass fraction gains would be massive. Get a filament winder, etc

Thinner Wall Case

This would be a much less difficult task than most other hardware redesigns, but would still provide decent performance gains. Improving insulation would still be important, but likely not as much as in a composite case. Unfortunately, unless a thinner wall is achieved by machining the outside of the case down, finding a liner would be difficult. This challenge could be overcome by 

Casebonded Motor

Attach the propellant directly to the inside of the case rather than using casting tubes and a liner. Provides better performance by increasing case volume loading. Requires new adhesive strategies and necessitates a monolithic design, which makes casting a challenge.