Text variables with text preparation:
Text variables whose final values ​​are calculated using a script, GREP, case conversion according to user-defined schemes, and the ability to assign character styles to parts of the resulting string would be very helpful.
Text variables can extensively use the existing serialization mechanism for paragraph and character styles, such as in running headings.

For example, many scientific journals generate a DOI (Digital
Object Identifier) for each scientific article, which is a URL that optionally uses a constant prefix, year of publication, issue number in the specified year, and the initial page number of the article. Rather than checking all these details each time, text variables with text preparation could be used.

It would also be interesting to work with non-text variables for color and size, which could be used in styles, changing many styles with one variable.

It would also be nice to be able to use the values ​​of multiple text variables in building a combined one.