There are so many elements of Rev6.8/CaH in particular that would have played out differently with Darius rather than Methos, and his dynamic with Duncan as a whole is very different too. I'm sure they could have done some very interesting things with Darius but I'm not sure they would have had the same impact on the fandom and fanfic that the combination of Methos + the Horsemen did.

Yeah, I'm completely with you on that. For one, we always knew Darius had taken a Light Quickening and used to be a villain; we even already met one of his old buddies. It wouldn't have been much of a shock to find out about his "bad old days" the way it was with Methos - either for the viewer or for Duncan.