It occured to me that expanding BoW on your own should be quite doable by using common wargaming 20mm scale miniatures, it's something I'm feeling tempted to do (though I'll of course see what FFG comes out with, it does seem like they mean business re expansions to BoW). So let's brainstorm a little. What unique unit types would be relevant for what houses?
I'll start with Targaryen, assuming battles take place during Robert's Rebellion.
I propose spitfires, pieces of artillery firing pots of dragonfire at the enemy. They're mentioned in A Clash of Kings when Cersei prepares the defence of King's Landing, and it's mentioned now and then that Aerys II was obsessed with fire and of using dragonfire for military purposes, so it isn't much of a stretch to assume that spitfires were around already by his time. The novels don't (as far as I remember) describe them in detail, so I'd assume it's something with a bow like a ballista, but firing a pot rather than a bolt.
What effect should pots of medieval napalm have? I'd think it's a weapon whose efficiency would to a significant extent be a result of the terror effect of sticky flaming liquid raining down on you. I'd suggest the following: The spitfire fires at a range of 6, rolling 2-4 dice depending on rank. Only flag and valour results have an effect. Each valour result inflicts one casualty (regardless of target rank; rolled shields have no effect). If at least one flag result is rolled in the attack, one additional flag result is inflicted (similarly to Last Hearth Lancers in Wedge formation). Furthermore, if at least one casualty is inflicted by rolling valour results, yet another one additional flag result is inflicted. This would make it a unit not very effective at killing stuff, but the more effective at disrupting formations and depriving enemy units of stalwartness. Finally, as a piece of field artillery, it should be appropriately cumbersome. Move 1 OR attack seems appropriate to me. With only two figures, it will be appropriately vulnerable to attack. It should further never be able to count as stalwart. Flag results rolled against it in melee would always inflict an additional hit instead.
Then, House Baratheon, another key player
I propose halberdiers, a unit that would both be able to make use of formations and fulfil a niche which no unit so far does: A counter to heavily armoured troops. The first formation would be spearwall, i.e. using the pointy ends of the halbers like spears. They gain the Polearms keyword. The second formation would be hacking assault (for want of a better term), i.e. using the axe blades. It would give them the keywords Advance and Armour Piercing. The latter is a new keyword of use against red rank units. If a struck red rank unit has Heavy Armour, it is negated. If it doesn't, it only counts as a blue rank unit for the purpose of determining damage inflicted.
I can't think of any more house-specific troops at the time, though there's a specific kind of unit type quite relevant in real medieval war that's missing from the game: Skirmishers of different kinds. While foot skirmishers were more relevant during the ancient than medieval age, light cavalry has been used for hit-and-run throughout the history of war. Green rank cavalry as currently in the game is only marginally faster than the other, and doesn't really have any ability to make a hasty retreat. Therefore, I propose the following keyword:
Engagement tokens are never placed between this unit and another (this means that it cannot be flanked). Whenever the unit is attacked (not counterattacked!), the first hit scored against it is converted into a flag result. Also, units pursuing it cannot make an additional attack against it (pursuers can move and attack other eligible targets normally). However, the unit can never ignore flag results because of any effect (stalwart, etc), nor can it ever counterattack. If it cannot retreat due to blocked retreat path, it suffers casualties normally.
On first sight it might look like skirmishers would be too hard to kill, gaining an effective cheap version of Toughness(1). However, it should be obvious that pretty much all skirmishers would be of green rank anyway, making it not too hard to inflict two simultaneous hits against it.
What do you think?