As many of us have indicated in this feature request forum already: although we like Simplify3d and enjoy using it, the infill is a huge weakness. This is the only portion of the program that is lacking, which causes me to not whole-heartedly recommend the software to others.
I would be satisfied with this simple fix:
Place a checkbox in the infill tab "Merge infill angles onto each layer"
By checking the box, whatever infill angles you specify in your angles list will be projected onto every layer. This will remove the problem of having stringy infill, in which the only strong points in the z-axis are where the infill cross-over the layer beneath.
Right now, the only way to get infill of decent strength is to "fake it" by increasing infill width. You can manage to get solid walls of infill this way, but it is only a band-aid. The layers are still not fully bonded by design (look closely at your print preview when you increase infill widths: width is increased, but the slicer is still bridging the infill over empty space).
Simplify3d needs to allow us to project all the infill angles so they occur on every layer. This way, the "infill extrusion width" setting can be used to do what it should: increase infill extrusion widths.