I enjoy setting courses out and my thoughts are as follows
If possible walk around the ground. Moving forward of the 'normal' firing line sometimes opens up new possibilities.
Positionals 25-50% are gettable the others test you.
Try not to put reducers together but spread out over the course
Subtle changes of course direction, such as over a few lanes.
Whilst it may 'seem' windless, open areas within a wooded area (that get some sun) can cause drafts to waft around as the air is heated.
Use the 'tunneling' effect with foliage, works well if shot through bright areas as well.
A high target that has no back ground behind it can cause issues. Especially if placed so when approaching the gate it has some sort of background but disappears when you sit down.
50 gallon barrels make a good noise when the target falls down. We have used then up in trees before
hole cut in the front target bolted at the back.
Different shaped face plates as well. I have seen lollipop or rectangular ones. With minimal faceplace can confuse the eye for range.
Short and long targets on the same lane.
I think every club member should at some point have a go at setting either a few lanes or a course out for a Winter league. They may see target placements that you had never thought of before and they can also get an idea of how hard doing courses can be.