>>6421629MORE NEW GETTER ROBO FUN FACTS
The "Getter future" episode is a slight modification on the "Encounter with the Unknown" chapter from the Shin Getter Robo manga (this chapter was first published in Getter Robo Generation 6/25/1999)
Benkei Musashibo, while obviously being a combination of pilots Benkei and Musashi, also is actually a three character combination. The third is Baku Yamagishi of Getter Robo Arc, who Benkei Musashibo shares an identical appearance to and has a similar background as a monk.
Speaking of Benkei Musashibo, the scene where he attempts to destroy the Getter Robo is taken from the Getter Robo Go manga, in which the titular character, Go, tries to blow up Getter after it drives one friend insane and kills another.
After the time travel arc, the New Getter Robo team is met by an upgraded Saotome lab equipped with missile launchers and other defenses. This design and scenario is actually from Getter Robo Arc, when an enemy hijacks the Saotome Lab weapons systems and attacks the Getter team. The lab itself was upgraded by Hayato when he rebuilt it (after the events of Getter Robo Go).
There's more references, but I haven't watched New Getter Robo is a few years. This is all I can remember for now.