This is a very neat idea.
This looks like it would scale quite well. How the opposing forces compare--who has greater numbers, who has the better position, who has the superior soldiers--will determine what bonus the players get for each Battle Move. For example, against the village militia, a squad of light cavalry might get +2 melee attack bonus. But against an ogre warband, the same cavalry might get no bonus at all.
What's really cool is you've departed from the usual Move structure of roll+(pre-calculated)stat. Here, the stat has to be calculated each time, largely depending on what established in the fiction. You've managed to load the comparison of HP, weapon damage, unit size, etc. into single stats. Genius!
I can imagine checklists to help calculating the bonuses for each Battle Move. Like for manoeuvre:
Do the player's forces have
- a fortified position or are they going against one
- access to faster units
- greater numbers
- unique access to certain types of movement (flight, burrowing, swimming, secret pathways)
This makes me think that the grim portents can really define the length of the engagement. So you could frame the grim portents to set up a siege situation. If player's were laying siege against a keep, depending on the player's plans, the grim portents could be:
1. Mines dug under keep
2. Keep defenses weakened
3. Keep surrenders
As such, some of the player attacks would involve undermining walls, likewise failed attacks could mean the defender discovering (and killing) underminers.
This is exactly the sort of thing I've been looking for for my fledgling DW campaign. Thanks for sharing!