I'd like to request an adjustable infill % based on not just height but also how far from the start of top layers.
For instance if you print a cube and don't need much strength you could get away with 5% infill. The issue here is that the top layers would droop due to the large gaps in the infill, needing more top layers to get a good surface finish. What I would like to see is S3D have an option to increase from a low % infill to a higher % as it gets closer to the top layer. So you might select 5% standard infill, but 40 or 50% as you approach the surface and S3D would automatically work out the best way to do the change and increase in infill as it approaches the top surface. Depending on how this is done it could actually allow a reasonable increase in print speed and reduction in infill plastic, and thus an overall reduction in plastic use without needing to adjust the number of surface layers to maintain quality.
Edit: While the cube could easily be done by adding more processes that change based on height, the big use of this would be where you have an irregular surface, such as a topographical map for instance where the ground gently rises across the print. The increase in infill would only be near the locations where there is a top surface being printed, and the rest of that layer would be a lower infill