A lot of people are saying documentary and I just want to point out that if this idea is too expensive it's not worth doing. A documentary would be cool, but I don't think anybody wants it if it's going to detract from the game. I imagine every backer would prefer that their money go towards making the game awesome.
I thought one of the most interesting things to come out of the Double Fine Adventure documentary so far is watching Schafer go back and play Day of the Tentacle. I think having some of the designers go back and do a retrospective on some of their older games in video or written form would be a cool backer reward.