HI Wampus, In my game it's 2D but in 3D it's the same only whit Z...
I explain how I did, I did a classe, and a machine State, in example the first movement it's a Senoidal, the second it's an Staright line, and so on... changing some values you can modifiy very easy this functions whit parameters, more or less complex... and you obtain derivates for each one.
Another way it's take an Editor 3D, comercial, I think the Special for Glbasic, Blender not, the other, I don't remenbe de name, and draw Splines, export this data into an array , and whit a counter follow this vertex what makes the splines... the trouble I think if the lines have a lot of points, the memory can be affected... or if you know how do it... whit interpolation betwen vertices if the Spline.
Another way it's do a simple editor, put 2 cameras from a side and from top, and drawing points and some maths, you can do a simple trajectory Editor.
Really not it's too much complex.
Uhmm Space Harrier... I burned my Joystick in Amstrad CPC, playing to this game... and I thinked a lot of time make something similar whit GLBasic, and don't see too much complex...
Regards Wampus , and if you need an explanation more in depth, comment me, and I try to put some code example.