Oh, just noticed the parters' site and got my diagnosis. I got Armadimon, a digi I've never cared much for like, at all. But I've grown on it while searching for a good line for him, and got a nice and interesting one, even if perhaps it has been done before.
Armadimon ->
Archelomon ->
Zudomon ->
Marsmon
The theme beign "going to the sea" and then "undersea volcanic eruption", so physical -> water -> water/electrical -> fire is somewhat justified. Also, I like Zudomon -> Marsmon because of its mythological implications; Zudomon could very well be seen as the equivalent of Thor, Mars is equated with Tyr via interpretatio romana, and Thor and Tyr shares a common PIE root, perhaps even beign the same god before the PIE split. Finally, the line is visually pleasant, as many characteristics from the previous evolution (mainly shell and mammal hide). Finally, I'd like to notice that Archelomon -> Zudomon is now anime canon, from Zudo's evo sequence in Adventure:, that besides that Zudomon HC card (Bo-1033) that lists it as evo req. Armadimon to Archelomon is canon via Bo-518, and Zudomon -> Marsmon is not atested in canon in any way AFAIK.