When it come to Mercenaries, we will hopefully be recreating the mercenary guilds from the Original SoD, so the 'old mercs' of M&B will be less prevalent as there will most likely be a whole lot of mercs available.
The Nords have no plan for a cavalry regiment, originally there was (I think merc guild) called the valkyries (Or something like that, that might have been a troop name) anyway these were placed as an 'inland' off shoot of the Nord clans, and had cavalry. We also had tlaks of the possibility of an 'anti-horse' unit for the Nords.
I always found fighting a siege against the antarians pretty terrifying, it was easy to clear them off the wall, but if their javelineers waited beneath the wall for you to climb over it, they could absolutely massacre your troops as you strung out in a nice long clump getting to the stairs down..
Personally I want to leave both out, until everything is redone and the dust settles, only adding in something more if it turns out that they are hopelessly over matched. The Nords are coastal raiders, horses take up lots of space, which means less warriors and less space for loot and prisoners, neither of which would appeal much to a Nord raider!
However the times are a-changing for the Nords so they may find the 'old ways' being washed aside by th demands of a 'modernising age'.
I like the Vaegirs too, I always use a double-handed axe on horse back, so I like their cavalry for that fact. They will be rounded out more, with some low level 'sword and board' style infantry, but otherwise they will be much the same as they are now (Hopefully).
The Khergits as you say, just need something to help them in seige dynamics, which will likely just be some toughening of armour, we shall see with that.
Right now this is mostly just 'speculative/conceptual' from me giving each faction a style and doctrine of war that comes from a wider Lore overall, with the logic of their identity feeding into their logic of warfare, then hopefully watching how they all shake out against each other.
My Hope is that we will end up with a rock/paper/scissors/lizard/Spock outcome, where each army will triumph and fall in equal measures depending on who they are fighting, so that no faction will be dominant over everyone but each faction will be different in how they fight.
The other thing that we hope to try and do, is change the basic mechanics so that you cannot have a whole elite unit army, as the AI never does, and pretty much the elite of anything will win out in a fight, so we will hopefully make the armies (and therefore the strengths and weaknesses) more balanced, again making it harder to be overwhelmingly dominant.