Idk, I'd like to see Rheged in that mix eventually... it's relatively powerful and positioned to allow the player to take the fight into any part of the map, giving a lot of freedom of play.
Dyfeint is pretty much stuck in the lower third fighting the Saxon/German invasion, good choice for a first campaign though, as precisely because of that, you won't have to have everything fleshed out 100%.
For the Germans, Saxons seem the obvious choice, but might turn out to be rather boring or require a lot of scripting/artficial and ahistorical stacking of the odds against a human player. They work better as the big bad AI boogeyman than as a player faction, IMHO.
Dal Riata for Irish/Gaelic sounds good, interesting faction in a challenging position that'll highlight a lot of the cool stuff the mod and setting have to offer beyond the standard "Arthur vs ze evil Saxons"