Realistically, these army packs would still be considerably cheaper than Bricklink.
I certainly would be interested, but I think that the price would limit my purchases to a few packs.
That being said, I would definitely like to see a medieval "army builder" so long as it included the right figs and accessories.
For me, an important feature would be decals. I would hope that the torsos were of a generic pattern (possibly a mail coat or something similar), so that they could easily be used with any faction. Similarly, shields would be blank, so that a variety of different decals might be applied.
Another selling point would be accessories. Frankly, I want to see arms and armour that go well together. Hopefully, a few new helms and weapons will be introduced. I know I'm not alone in my desire for an official basinet or 15th century sallet. Both of these helmets could be used both for knights and infantry, and look good with or without a visor. A new sword, and a smaller halbred would also be most welcome.
When it comes down to it, the most important thing is that the arms and armour appear medieval, rather than futuristic. I would also expect the armour be in conventional colours (various greys/blacks). If this was the case, I could definitely see myself picking up 10+ of these packs.