At ~all times for a long period of time during Gitmos operation, there was at least one (revolving) prisoner that no nation on earth would take. I think that was the biggest challenge for someone who actually wanted to close gitmo, to close it. Not clear where you would put them that wouldn't be yet another prison.
I guess now that the US has normalized relations with the Taliban, maybe they'll end up sending them to them, not sure who else will take the last ones.