Minerals —> Gas
Another resource mechanic is a modified version of the original gas mechanic. Instead of getting a flat refill of gas for a flat rate of minerals, the effect would degrade each time you use it. This degradation would come in one of three forms: either less and less gas for the same amount of minerals, the same amount of gas for more and more minerals, or a mix of both. In between these "phases" the player can obtain fumes.
The big question is, should there be a fixed maximum number of phases? Obviously, if it is set to return less and less gas, there would have to be and end at some point; however, this wouldn't necessarily be so if you can get the same amount of gas for an ever increasing amount of minerals. In theory there would come a point where additional gas would just not be worth the minerals anymore, and it would be cheaper to expand instead, but this would be a natural limit as each player would have a subjective value on gas.
The big advantage of these two resource mechanics would be allowing players to explore different strategies and build orders. Players who are gas heavy (high tech and casters) or mineral heavy (low tech and warriors) could use these mechanics to really make their own gameplay, and thus make it more fun and interesting.
Of course a very important balance issue would be to ensure that, even with a perfect back-and-forth between these mechanics, there is a net loss of resources. One should not be able to rapidly switch between mechanics in order to gain more resources than someone who never uses the mechanics at all. If you pitted a person who never used them against a person who used them at every chance possible, the latter person should only end up with about 85-90% of the resources that the former person does after the same amount of time. The mechanics are for a quick boost, not continuous abuse.