Here are my first few:
1) Obvious stuff: support fins attached to fins, pods, tail rings, etc.
2) Don't crash on Mac when switching away from 3D views.
3) There should be a couple of state settings for each component, easily accessible in the main tree view:
a) Omit from simulation: some parts are there for appearance only, and they can confuse the simulation terribly. It would therefore be incredibly helpful to be able to designate certain parts as being for appearance only, and not included in the simulation.
b) Disable: sometimes you want to have a couple of variations of a part (e.g. perhaps you are experimenting with fin shape). I want to be able to completely disable some parts, so that they neither appear nor are simulated.
4) Right now, the appearance of each component is set individually. This makes it incredibly tedious to change theme colors. Let's say I have a red, white, and grey rocket like this:
It has 6 grey components and 2 red components. Now I want to change that grey. Well, I have to edit 6 components and change them all to the same new color. That is a ton of pointing and clicking, and it doesn't have to be that way. Instead, within a design you should be able to define a set of appearances, and then simply refer to them for each component. For this example, I'd define three theme appearances, for white, red, and grey, and I'll call them theme1, theme2, and theme3.
In the appearance editor for each component, I'd choose to select from available theme colors, rather than defining the color, shininess, etc. Now, if I want to change the grey to something else, I go and edit theme3, and after I change it, all the grey parts change automatically. Playing with color schemes now becomes easy and fun, rather than tedious.
That's enough for now, I'll add more in my next post.