LaughingTulkas
Chieftain
Just stumbled across this thread and thought I might throw a couple ideas out there. Your mod, your rules, but here's a couple things you might consider instead of Field Marshal:
Legendary Commander
Warlord (or High Warlord)
Strategos - old greek rank "army leader"
Polemarch - from greek again "war lord"
High Constable - medieval rank, but I don't like this one
Personally, I kind of like the greek ones, but if they are too greek, I think the simpler "Legendary Commander" is a bit more in the fantasy realm than "Field Marshal," especially since many of the factions simply don't have the social and political structure required for such a formal, hierarchical rank.
Legendary Commander
Warlord (or High Warlord)
Strategos - old greek rank "army leader"
Polemarch - from greek again "war lord"
High Constable - medieval rank, but I don't like this one
Personally, I kind of like the greek ones, but if they are too greek, I think the simpler "Legendary Commander" is a bit more in the fantasy realm than "Field Marshal," especially since many of the factions simply don't have the social and political structure required for such a formal, hierarchical rank.