Not sure if anything like this exists but I think it would be really amazing if a file manager would allow to index specified files and folders and while navigating to any of the indexed locations, it would allow to perform instant content search only showing results for that particular location. Kind of like file/folder name filters in Directory Opus or XYplorer but for content as well. I am surprised that big names in this field like Total Commander or the two previously mentioned don't have this functionality. It would make document management so much easier and appeal to a larger audience. Maybe developing a search engine from scratch is a project on its own but why not provide this through a plugin and the end user would have the choice of purchasing the additional components. For instance, I think DTSearch could be integrated in such a way.
Also, a similar approach could be done for other features such as internal file viewers (i.e. Oracle Outside In) and rich text editing.
Although still needs to mature and currently doesn't have indexing, there is already a very interesting project called TagSpaces that is taking a modern approach to file management. I hope other file managers will take example and start thinking towards the future.