So since some of the Death Generals were revealed to be ultimates here are my ideas that just came to me via brainstorm:
Grumblemon, Sethmon, Kinkakumon > Gravimon
Ranamon, Depthmon, Sealsdramon > Splashmon
Arbormon, Yasyamon, Petermon > Zamielmon
Gravimon > EBEmon
> Pharoahmon
> Queen Chessmon
> NidHoggmon
Splashmon > Metal Etemon
> Hi-Andromon
> Surfymon
> Aegisdramon
Zamielmon > Parasimon
> Zanbamon
> NoblePumpkinmon
> Argomon
Gravimon + Zamielmon = Tactimon
Zamielmon + Splashmon = Vulcanusmon
Splashmon + Gravimon = Duftmon
I'm treating the three as a trio. I'm not so bothered by them not being the same level as the other death generals since I already headcanon Water, Earth, and Wood attributes as being second tier to Fire and Metal (based on Legendary Spirits).