Since "Presidential Debates" are wholly a construct of television, they are held wherever TV decides to hold them, whenever TV decides to hold them.
After all, they are and always have been nothing more than a television show.
I'm not even sure what the point of them are these days. Between Trump and Hillary, there can't be many undecided voters who will be swayed by the debates. People already know who they'll be voting for. Now it's about getting them to the polls.