Actually, I would place the “Subscribed” button as another layer, or remove it completely since it might only make sense to filter subscribed communities… (does it?)
Also I would add a “New filter” option to the menu.
I understand this change would need a great deal of work to be implemented since to create a filter one would have to have access to several filtering options: add one or several communities to search in, add keywords (or hashtags - but, please, see @mark@programming.dev’s answer) to include, maybe certain users…
Would it be realistic to implement some kind of language like: “keyword -unwanted AND (keyword OR keyword) AND community:!comunity@instance AND user:@user@instance …”?
Maybe bookmarking the post and remembering to get back there later as a workaround?..