The design issues remaining for backpack stoves are fuel efficiency (reducing loss of heat)and wind protection (which is a big factor in heat loss). Some pot/stove systems have a heat exchanger built into the pan, but these are still quite heavy. I would think a good project would be to accurately measure heat loss (in various conditions, cold,windy, sunny, shady, etc)for several of the commercially available stoves out there, and then design gadgets or stove modifications that significantly reduce the heat loss. Look into such things as burner size vs pot size - what match has the least heat loss. Another factor, pot shape - does a tall skiny pot heat water faster than a wider lower pot?