Godannar/WMG

Kouji and Shizuru got together later and started a family of badasses
All their descedants were sharing his Hot Blood and Haminess and her Badassery and Catch Phrase. Last member of their dynasty was Kamina.
 * Another famous member of said dynasty was Crazy Steve also known as Goddamn Batman!

Kouji's parents somehow knows Mazinger team
That's why he's named after Kouji Kabuto and Tetsuya Tsurugi - his parents were their friends.

Kouji's parents are pair of Otaku and Go Nagai fans
If Nicolas Cage could name his son Kal-El then I bet two okatu could name their child Kouji Tetsuya.

Show takes place in one Universe with Gao Gai Gar
In episode 2 Go is reading about wedding of "Gai and Mikoto" in the newspaper. While it wasn't ever stated in the show, it's possible that Gai and Mikoto could get married between the events of Gao Gai Gar and FINAL. And wedding of guy who defeated Zonder Empire clearly would make it to the front page.

Godannar is the distant past of Gurren Lagann!
The Mimetic Beasts and then the Insania Virus were sent by the Anti-Spirals. When THAT didn't work, they decided to take a direct approach.
 * Further, Shizuru and Kouji could be the ancestors of Kamina while Go and Anna could be the ancestors of Yoko.