I am dying to see a manga where Hashiras are teaming up (Well, there is the prequel manga, Stories of Fire and Water, but it's only Giyuu, Shinobu, Rengoku, and a Mizunoto-level Mitsuri), especially this duo. It has been said in the manga that these two respect each other's skills in battle, likely hinting that there is a possibility that they had worked together one time or another...
If Rengoku lived and tengen didn't get injured and they teamed up. They would be unstoppable. Just imagine Tengen's fast flashy and destructive moves in combination with Rengoku's first form or his Final form. The team name would be Tengoku.