I just had the idea that you could give special resources more meaning by being able to spend them for temporary effects. Something like "spend 1 harmony crystal to boost approval for 10 turns by 10%". Those could be projects like "Aid Economy" whose appearance depends one the availability of the resource and certain tech prerequisites with a "built time" of 1. As long as resources are available one could select those projects several times. They would then come into effect one turn after the other, but since their operation time is bigger than 1 turn one could still kind of stack the effect. In above example with the Harmony Crystals if you would select the project three times you would get 10% approval boost after 1 turn, 20% after 2 turns, 30% after 3 turns, then it would remain at 30% for 7 further turns, after 11 turns from the start the boost would decrease to 20%, after 12 turns to 10 % and after 13 the effect had worn off completely.