There are so many, but let me try to list a few.
Religious figures (eg. Jesus, Mohammed, etc) and ask them their reaction to how their names and teachings are currently being used and interpreted.
The American Founding Fathers and ask them for their viewpoint of current political discourse and if their intentions and values are being portrayed accurately.
George Orwell, because I'd like to ask him if he thinks his predictions in 1984 and Animal Farm have come to pass.
Henry David Thoreau, because I'd like to ask him what advice he'd give current generations.
There are many more, but I'll let those suffice.