I think it would be neat, once costs are implemented for parts, to have a hybrid system where you can spend a little science to research different efficiency upgrades for parts, and then you can spend a scalable premium to upgrade the quality of certain attributes on a part by a certain percent (up to a limit). Being able to spend a premium to decrease weight, increase thrust, or boost fuel-consumption efficiency would be killer for really tight rocket building. It might also make it feasible to get ship designs working that are just under the efficiency curve with current parts - but you'd have to pay the extra. Right now I'm trying to make a SSTO dropship spaceplane that can also hover and land vertically to pick up mining base components and move them around. Besides fighting with the thrust-vectoring issues to just keep it in the air and controllable, the biggest issue I'm running into is that I'm just below where I need to be on the TWR and fuel consumption curve to make it work. The vertical ascent engines can lift it but eat too much fuel, and the forward flight engines (using RAPIERs right now, tried a few others) don't have quite enough thrust.