Another apparently obvious filter is the development of advanced nanotechnology and AI. However...
Although I'd immediately agree that those are filters hindering the spread of any humanlike civilisation, it doesn't at all stop the spread of other, technological forms - the "vile offspring", to borrow a term.
Honestly, I have no idea what might work as a future filter, at this point. I don't think the expense could be it; that's going to drop, quite abruptly once uploading (or straight-up AI) works well enough.
Although I'd immediately agree that those are filters hindering the spread of any humanlike civilisation, it doesn't at all stop the spread of other, technological forms - the "vile offspring", to borrow a term.
Honestly, I have no idea what might work as a future filter, at this point. I don't think the expense could be it; that's going to drop, quite abruptly once uploading (or straight-up AI) works well enough.