I think that the easiest way to do this would be to use particular. You could animate the path of the emitter and then set the particles per second to your frame rate. Then adjust your size over life and the life span of the particles to get the look your are trying to achieve.
Here is a link to a tutorial you can get more detail about the process of making a dotted line with particular.
I think that Trapcode 3D Stroke will do the same thing, and it has a taper modifier built in. I think that if you just use the "Adjust Step" value under the Advanced tab, it will give you the adjustment of the "brush" spacing to show as dots.
create a solid, and use the pen tool to create a path in the shape of the dotted line. then select the mask path property and copy it (select the actual property called mask path, by twirling down the mask properties).
add the circle effect and select its 'center' property (again twirl down the properties to select it) and paste the path into the center property. you should have a circle that now follows the path.
key frame the radius property so it tapers as needed.
add the echo effect after the circle effect and adjust the echo time and number of echoes to create a string of circles that move along the path.
if you didn't want the string of circles to move along the path, but rather be static but write-on, you can try this expression in the both circle's center and radius properties: offset = Math.abs(effect("Echo")("Echo Time (seconds)"));
t = Math.floor(time/offset)*offset;
Great suggestions! 3D Stroke is great for that but here's another way to do it inside AE. I"ve used it for animation dots along a path which also animates like a waggling cable.
Create your stroke or path in Ai.
In AE, context-click on the Ai path layer and "Create Shapes from Vector Layer".
In the new vector layer (which will show up at the top of your comp [type "1" to select]) twirl down to where it will say "GROUPS", highlight that and click on the little "ADD" button then add a STROKE.
There you can specify dots, dashes and animation on the vector layer and your Ai will still be sitting below but muted.