Varying infill percentage based on distance from walls, feature size, curvature, etc.
Look at the way any FEA meshing software works and you will know what I mean. Especially Cartesian 3D meshing.
Distance from walls is by far the easiest and I believe robust enough method to determine infill density.
For example you can specify minimum and maximum infill percentage and gradient. The software then computes infill percentage based on criteria as mentioned above.
Another approach is to be able to import 3D mesh files and map the mesh density to infill density.
I believe this would be far easier to implement as there will be no need to work on meshing algorithms. Just use other meshing software, export/import in a widely supported format and generate the infill based on mesh density. Not a standalone solution, but better than nothing.
Either way the resulting infill should achieve both material and time savings as well as stronger prints.
Very advanced code for 3D meshing has been available for years now, including free open source solutions, and I'm really puzzled as to why no slicer implements such a feature to date.