So despite the "not in my back yard" attitude which prevails in most states when they envision terrorists as guests in their prisons, there are places which are desperate enough for federal dollars and jobs to take the terrorists, so housing them in the US is certainly feasible. However, aside from the technical difference of being on American soil, a supermax prison devoted solely to GITMO prisoners would not be much different from housing them where they are now. They would still be isolated from other prisoners, likely in a very remote part of the country, and held under uniquely high levels of security. No one has ever escaped from a supermax prison in the US, but keeping the prisoners secure is really the least of the problems.
But even if we put them in prison somewhere else, respect for the rule of law and the Constitution demands that we give them fair trials. Yet there's a reason why the Bush administration was only able to try a handful of them. The evidence against the rest is strong enough to convince most people that they really are dangerous terrorists but it is not sufficient to form an effective case good enough to stand up in US courts which have already rejected the kind of evidence the government is trying to use in many of these cases. There's enough evidence to know they are the "worst of the worst" but it's often not the kind of evidence which is up to the standards of a normal American court. Twenty reliable sources may have told us someone is a terrorist and we may absolutely believe them, but without witnesses to acts of terrorism or physical evidence, a trial may well end in an acquittal which should result in the release of the prisoner. Then what do we do?
We can't keep him in the US because he's not a citizen and he really is a dangerous terrorist even if not convicted, so he's not about to get a visa. We can't send him home or to another country because they know who and what he is and won't take him. We can't just release him in the wilds of Waziristan when no one is looking, because he'll just start killing civilians and US soldiers as a recent Pentagon report demonstrates. What do we do that honors our legal system and our Constitution and also protects our people and the world?