Hi,
@Axalon: I myself never understood this differentiation between major and minor factions. Already in Vanilla I was annoyed not being able to play Novgorod in early, for example. My personal opinion is: If it´s on the map, it should be playable (except the Pope maybe, for his special role in the game). In Redux, for example, I´d like to have a go with the Portugese, starting with a single province and quite some challenge on the doorstep. Or the Hungarians, stuck in the middle of hell between Germans, Italians, Polish and Byzantium. Being a rather poor modder, I´m at loss there. I enjoy the game anyway, as you might know ;-))
@Zarakas: Running danger to annoy, I say again: My all-time-favourite faction are the Danish/Norse, optional factions in both Vanilla and Redux (never thought really about it, just played them). And for me, my personal style of playing, my preferrence being turtling around till I strike fast and hard, they are simply the perfect faction. In Redux, the Russians as well, and the English with their ranged warfare, are a treat for themselves.
To make the Byz some more challenging in Redux, maybe a way would be to let alone the Saracenes for a while till they tech up. If that´s the case, they might become the challenge you seek for the Byz, especially in desert warfare, where those heavily tanked up ByzCav exhaust themselves already just standing around in the heat ;-))

Wish you much fun playing,

greetings daigaku