I know some have already attempted this and other variants are already out there.
Here are "my" ideas of how it would work to integrate parts from HQ into BM (instead of the other way around as many have also done already). Someone has probably already thought of these and can point out any issues...
And this has obviously not been playtested yet, heck I haven't even played the real game yet! But I'm still thinking of how it might work.
I do like the idea that some units will be carrying treasure chests and these could be captured by another unit destroying them.
I also like the idea of capturing enemy units somehow and forcing the other side to ransom them back, or hire mercenaries for additional units, but that part isn't well thought out yet.
So the Four Heroes would be able to join the battle. Perhaps having all four of them on the battlefield at once would be too powerful, unless the bad guys also had four evil "heroes" on their side as well (perhaps the Wizards of Morcar). But even having one Hero on the Imperial side and one Chaos Warlock on the Chaos side would be cool.
Unit substitutions might be in order. I would love the idea of putting in Dwarven and Elven units, but I'm not made of money, I don't have access to a 3d printer yet, so maybe start small here. Even subbing in 3 Fimirs per base in place of the Beastmen would be cool even if their stats were identical.
"round" would mean by the next enemy card draw
Each Hero may individually "tag along" (or piggy back) with another friendly Imperial unit (except the Mighty Cannon) (Idea freely stolen from the first person who posted it over at boardgamegeek, sorry I forget your name)
Each hero should choose a unit at the beginning of the campaign. Should they wish to change units, they may only do so on their
unit's turn to an adjacent friendly unit as a free action. Their bonuses then are transferred from the old to the new unit, but damage does not (presumably they weren't the one who was injured). This means though that if the unit is down to its last body point, they can't leave, because that implies that unit is "dead" without the bonus.
Adding a Hero confers certain benefits. Here we presume these are veteran heroes, so they are more adept at magic, and are battle hardened.
Barbarian - adds 2 BP and 1 extra attack die to the unit. May re-roll up to 3 dice.
Dwarf - adds 2 BP and 1 extra attack die to the unit. May re-roll up to 2 dice.
Elf - adds 2 BP and 1 extra attack die to the unit and may use spells (skills), three of them.
Wizard (Alchemist) - adds 1 BP to the unit and ability to use spells (skills).
Spells may be cast on the Wizard's own unit, or another unit that is up to 3 hexes away (doesn't have to be strict LOS, so it can go "over" another unit). Gets the choice of 9 spells.
Ball of Flame (2 BP damage; target rolls 2 BM dice, damage negated for each blank rolled in defense).
Fire of Wrath (same as above but 1 BP damage, 1 BM die)
Courage (Wizard's unit or adjacent unit gains 2 extra combat dice in attack for as long as it is adjacent or in range, as in the case of ranged units, of an enemy target) -begins the next time that unit is able to attack
Water of Healing (targeted unit regains up to 4 lost Body Points). Brings Wizard's unit back if it is destroyed and spell not yet used.
Sleep (target must roll one BM die for each of its Max body points immediately and on future turns, if a shield is rolled, the unit recovers; if not it cannot move, attack or defend itself until the spell is broken)
Veil of Mist (affected unit may safely pass through another unit if it has the movement available to do so) (would only apply if they have the charge going) - alternately, the unit would not be target-able by the enemy on the next round only.
Tempest: the affected unit loses a turn (ignore next card for that unit)
Swift Wind: affected unit is able to double its movement (2 or 4 squares in the case of charge)
Genie: attack unit up to 3 hexes away with 5 BM dice (defends as normal)
Rock Skin: affected unit gets 1 extra defend Die until it sustains at least one BP of damage
Heal Body: restores up to 4 lost BP to a single unit (or brings your own unit back if you die)
Pass through Rock: allow unit to pass through a hedge barrier or fortification on your next move (if there is room on the other side and in the case of the tower, if it is unoccupied)
Not sure how the WOM spells would work, although a few would be easily adaptable like Arrows of Night if treated as a ranged 2 dice attack
elf spells?
twist wood (turns archers into melee units; ruins lances? 1 less attack die for knights)
haven't thought about the rest
Chaos Sorcerer and Wizards of Morcar are evil "Heroes" who can piggy back on friendly units (except the Ogre Champion)
They each add 1 BP to the unit and the ability to use spells.
Perhaps to make it more fair, if the Wizard or Elf plays, there should be two of them on the enemy side instead of just one?
Chaos spells
Ball of Flame (same as above)
Tempest (same as above)
Escape (teleport the unit to another square on the battlefield adjacent to any other friendly unit)
Rust (reduces a sword using unit to 1 attack die OR or reduces helmet having unit's defense by 1)
Command (take control of an enemy unit and may move it when that card comes up unless they break free)
Fear (reduces unit to 1 attack die until they break free)
Firestorm (hit one area of four adjacent hexes up to three hexes away; all within it EXCEPT the casting unit, roll 2 dice, for each blank, damage is reduced by 1 point but each are guaranteed 1 BP damage)
Lightning (shoot in a straight line bolt that travels three hexes and every unit in the path takes 2 BP damage)
Cloud of Chaos (sleeps all enemy units in a four adjacent hex area up to three hexes away)
Summon Undead: brings in undead reinforcements to adjacent hex
skull = 1 skeleton unit
blank = 1 skeleton and 1 zombie unit
shield = 1 skeleton, 1 zombie and 1 mummy unit
(here you would need ROTWL to have enough undead figures)
Summon Orcs: brings in orc reinforcements to adjacent hex
skull = 1 orc unit
blank = 2 orc units
shield = 3 orc units
(you'd need KK to have enough orcs)
Undead stats:
5 skeletons: attack 2/defend 2
5 zombies: attack 2/defend 3
5 mummies: attack 3/defend 4
undead not affected by sleep spells BUT chaos sorcerer can be put to sleep, meaning he can't use spells again until freed
(they carry him along and protect him, but if entire unit dies, he dies too)
other chaos magic?
Ogre Mercenary
(regular ogre on a platform) 3 BP, 3 Ogre cards (reduces to 2 and then 1 then destroyed)