First, I am amazed by your plug-in, it's teriffic.
I only have a few wishes:
1. I wand exclude objects plus splines instead of only one spline.
The reason is that we have here a lot of big architectural scenes with a lot of scattered (by hand, must be) objects throughout the scene which shouldn't be intersected by Forest scatters (plants/humans).
So instead to draw a lot of small exclude splines it would be a lot easier to simply select all my non-collision abjects. Bounding box checking would do it.
2. What about collision checking not only inside a Forest Pack Object, but over multiple.
There are situations where we need multiple objects for layering different layers and they can't check collision with each other.
3. Unique Transform settings for every subobject type. We have a lot of things which must not be randomly rotated (cars) , mixed up with random rotated scaled people or small plants. I would split it up in two forest pack objects, but then they do not collide with each other any more, see 2.
4. A "make Forest object from selection" tool. We have to use handplaced or placed by data, expressions, object paint, instances and would like to convert such an arrangement with many different types of objects to a Forest Pack object to speed up scene handling.
5. Under custom edit, a better place object function, so instead of placing single objects you could paint many at once by drag over the surface. All customizable with brush width, density, sub-selection of your Geometry Ids, custom scale automatically orient to the drag direction and so on.
As you can see the most points are somehow connected to each other, solving one would help with others.
I think especially point 5 would be very interesting for a lot of people, who are tiered of using the sometimes crappy script object painter in 3dsmax.