A place to manage bulk style changes across paragraph, character, and object styles.
Probably not quite what you're envisioning, but as someone who works with CSS/SASS a lot and is used to making styling changes via a code editor, I've often wished for a more advanced way to get into the nitty gritty of indesign's style sheets. For complex documents, endlessly clicking through tiny panels to edit styles gets very tedious. And we are very limited by not being able to apply multiple styles to the same character/paragraph as mentioned in other comments.
Overall I would like to see indesign’s styles totally re-worked so that they can function more like CSS, or—dreaming here—actually be fundamentally rebuilt/remapped to their equivalent properties on the platform of standard CSS. Then they could be natively exported to/imported from a standard CSS file.