Firebrand - Hi bro, nice to see you too.
First suggestion (Fixed (anchor) sprites) - Thanks for the answer, I have already thought of that myself, having patches almost entirely invisible (only an object in it like a rope with wet clothes) and define its height so it can be used to watever reason (in the example, to be hanging near someone's window, or between two near buildings). Still, unless difficult to implement, the fixed sprites have many advantages over invisible, passable linedefs:
a) It allows the player to pass around it's borders (or below, above, etc), but it COULD be solid and act as an obstacle in itself (and thus, it could be destroyable, have scipts or specials based on hits attached, etc);
b) Sprites are considerably easier to add in a map, having a detailed tree top like in the pictures would require a lot of sectors only for it. In maps based on forests, the amount would be huge.
c)While sectors over sectors are taboo, detailed vegetation would be made much simpler with fixed sprites because you could have many of them above/below each other without any problems (for example, you could have a lot of plants on the ground, beneath a big tree full of branches, and nothing owould overlap or intersect).
Second suggestion (patch-over-patch sprites): I think it cant be done by sectors trick, so the feature request remains.
Third suggestion (3d sprite): this i guess might be hard, but still I think this would be extremely valuable for detailed city recreation. What I'm asking is: imagine a rectangle sprite 40 w x 120 h units. You could set its XY orientation (not only N or NW, but by the unit), and then the Z angle. If I make it have a 45