Question

the headings and subheadings of the body of an outline should be __________. long parallel interesting complicated

Answer 1

Thea headings and subheadings of the body of an outline should be parallel.
They don't really have to be long and complicated, and of course, interesting.

Answer 2

I believe the answer is: Parallel

Here is an example of parallel placement:

HEADING

SUBHEADING

The purpose of such arrangement is to ensure that the readers understand that the information that written in the subheading is a part of the heading as a whole. Also, from aesthetical point of view it make the writing become much more convenient to read.