Easy answer for me: Bill Cullen's "$25,000 Pyramid." All 150 episodes exist, and we'd get to see stars like Penny Marshall, Joan Rivers, George Hamilton, Kate Jackson, Frank Gifford, Rue McClanahan, Peggy Cass, Betsy Palmer, Robert Urich, Rob Reiner, Garry Moore, John Ritter, and Dick Clark (!!!) play.
As far as things GSN already has in their possession, I'd be very happy to see "$20,000 Pyramid" back on the schedule.
Brendan