"Ryu Hayabusa" is the main character of Tecmo's Ninja Gaiden series, and he is the mascot of SDQ because if DQ doesn't close, hopefully a NG machine will appear in the Replay Zone.
"Treasure of the Incas" was an old DQ attraction when it opened in 1998 - you drove a remote-controlled toy truck through a maze. You drove it by a video screen that had a POV camera of the truck, and the maze was covered with glass so friends could see the trucks and yell directions to you. (such as "NICKY, GO BACKWARDS!") In the first year of operation, it gave out tickets to treasures you collected. It was closed in 2005 when one of the trucks caught fire, but the mazes remained until 2007. When DQ gets updated, Treasure of the Incas must return.
"Zaxxon" is a 1982 isometrical shoot-'em-up by Sega. You fly your ship over walls and try to defeat the titular robot. A Zaxxon machine is at DQ, and we are trying to save it.