That’s it right now. I submitted some ideas around this that you could vote for:
(@revathimenon and @Kenneth R that second one probably needs to be merged into the first)
And there are some others around permission setting. But I don’t expect any of these to be delivered any time soon. A revamped search experience is coming soon and it’s not included there; I suspect additional enhancements around search will pause for a bit after this.
However, a few months back I was playing with this a bit and there seems to be some opportunity in the content field. The API will accept up to something like 10K characters in the key value pair for that field. Which then the search algorithm would use in indexing it for search (I think). However, most of those characters are never visible to the end user, as only the first few dozen show in the search result, then clicking the result opens the URL.
So, if you wanted to add some meta data, you could append whatever you want the algorithm to index to the content field and, if my hypothesis is correct, it’d help with search optimization for that content. But it would not work like Public Tags or anything like that; those filters won’t apply to federated search content.