Certainly, there must be some way to somewhat create a list of words suggested to be included in an index.
[Tex/LaTex] n automatic process to create index creation
automationindexing
Related Solutions
Do I really need to go over everything and add
\index{}?
Unfortunately yes. At best even if you write a script to automate it you will end up with a concordance, and a concordance is not an index.
In my opinion it is actually better to postpone the writing of the index to the time the book is almost ready. Writing an index is an art and most publishers employ "human indexers" to write an index that is useful and serves its function.
Since luckily, you missed it the first time round this is a good time to give it a bit of a thought and planning, before you delve into it.
The most important points to consider when developing an index, is categorization and consistency. Think of the likely readers of your book (or even the older you that will forget what the younger you wrote) and provide headings that are likely to be used when searching for information. Consider for example a historical book, describing early ships and their trade routes. It can be meaningless for example to just index the ship name by one word. Consider the following MWE:
\documentclass{article}
\usepackage{makeidx}
\makeindex
\DeclareRobustCommand{\ship}[1]{\textit{#1}\index{Steam ships!#1}}
\DeclareRobustCommand{\AUports}[1]{\textit{#1}\index{Austalia ports!#1}}
\begin{document}
One of the early steam ships to sail to \AUports{Melbourne}
was the \ship{Africa}. Its maiden trip was on the 1.1.1870 and
its last trip ten years later on the 13.12.1880.
\printindex
\end{document}
I have used a heading to categorize the ship as a steam ship (you can add macros as necessary) and provided a second one to classify Melbourne as a port. By creating a number of commonly used categories around your topic you can also ensure that you have a good classification system as well as providing consistency. As you might have probably noticed the ship's name is typeset in italics, to comply with the Oxford Style Guide and by creating a macro both the indexing as well as the typesetting are done correctly and efficiently.
Best Answer
I suggest you look at the script make-index.py (and related files) in the scripts folder of the download page at the Stacks Project (http://www.math.columbia.edu/algebraic_geometry/stacks-git/). The index it generates isn't really ideal, but at least their strategy will give you some idea how to get started. They seem to take the approach that (in a gigantic math textbook) the things which most deserve to be in the index are the italicized word(s) or phrase(s) in each definition environment. In my experience using math books, the most common reason I look something up in the index is to learn its definition, so this seems appropriate, although maybe not for books in other subjects. However you might be able to use the Stacks Project script as a guide to automate the creation of an index which suits your own needs, even if they are very different.