I have a few ideas for story missions, because I want to make a small campaign for my Orruk playing friend.
(Hey Mesandres, if you are reading this: GET OUT! This thread contains spoilers!!)
Because of that most of them are for Destruction, but you might be able to adapt them to another faction. My idea is to make a cool campaign for him that starts with a small part of his mixed Destruction army, and he has to recruit (unlock) his other units in the first few small missions. That will also give him an incentive to make use of his less-favoured units. Then, after he is finished recruiting, his army will attack the forests of Lustria/whatever dealing with Seraphon forces and/or Sylvaneth and/or Undead or whatever models I manage to have finished until then.

As a former D&D game master I like to come up with ideas for stories and adventures and I don't mind playing the enemy that will most likely lose the battle. It is about story here. You play to lose. But you want to make it close and atmospheric.
Keep in mind those are just ideas, some of them will probably work some won't, and I don't have planned more than that so it is hard to tell in which context they might fit. I think most of them will probably work better for small battles though, like 500-1000 points.
Ok, so here they are:
1. An Aleguzzler Gargant with a hangover runs around destroying stuff. You don't want to kill him though, you want to calm him to recruit him. So a part of your army has to damage him and/or run away from him, "kiting" him around, while a few others of your units have to attack/trick some drunk Dwarves/Goblins/whatever and steal their beer barrels. Once you have them, bring them to the Aleguzzler and he will drink them. Then he will join your army.
2. A big monster (friendly for you or not, maybe at least preferring to attack its captors instead of you) is trapped in a steel cage or chained to the ground inside or near an enemy camp. There are only a few guards there, but they can send a messenger running away, and if he reaches the end of the table he can call reinforcements. The idea is that the attacking player must prevent the messengers from doing that, and/or free the monster before it happens.
You have to balance the mission so the player who attacks has a disadvantage against the reinforcements, but with the monster fighting for him he would have a slight advantage.
3. Similar to 1, a monster is running around but you are not allowed to kill it. That's because you need it. Kite it onto a platform that can only be triggered by its enormous weight. That opens a door so you can get through. This is more of a dungeon-type game idea.
4. A map with a huge chasm, the player has to get to the other side. No flying units (or only a small amount) allowed in this mission, units can not be set up on the other side of course. So no Chamaeleon Skinks over there either.
- variant A: There is a drawbridge (closed) that can be lowered from the other side, but the lever is guarded. There are Arcane Ruins on your side with some enemies around them. You can attack those. Once you have control over the Arcane Ruins you can summon Spirit Hosts. Those can fly over there, kill the enemy, and then you can lower the drawbridge and pass the chasm.
- variant B: Same situation but without the Arcane Ruins. The enemy force on the other side has a Necromancer or something who summons troops to attack you. On your side there is a Witchfate Tor. You have to conquer it, use it to learn the spell from the Necromancer, and summon stuff over there to win.
5. Similar to 4. There is a big field ov Lava in the way instead of a chasm. Units can cross it, but receive some wounds doing so. So you have to balance it a bit, use healing or so. Maybe you can also get an artefact somewhere that allows one unit to cross it, and then that unit has to get another of those artefacts on the other side.
6. A guarded convoy moves through the area. You want to attack it, but you have to be quick. Every enemy unit from the convoy reaching a certain edge of the table gets away and cannot be killed.
7. A normal battle, except there is some special piece of terrain somewhere to the side of the battlefield, a player can conquer it. Examples:
- Arcane Ruins, you can summon Spirit Hosts there
- A static Balewind Vortex that your wizards can use to buff their spells
- Instead of terrain there is a neutral wizard randomly attacking one of the armies. If he is killed and any wizard gets to his position you can learn a powerful spell from his spellbook (such as Kroaks Meteor, to have an incentive to get him).
- A holy place that grants 1d6 insights (re-rolls) like a Starseer does, for the army controlling it.
8. The player wants to recruit a Savage Orc or Goblin tribe. They say their god wants them to stay and not join the army. Their god is a Dread Saurian/Dragon/other huge beast, maybe controlled and helped by a small group of (insert favourite enemy here). The player's force has to attack the god and kill it, becoming the "God-Eater", which of course makes the tribe join them.
9. An assassin hides in units and leave them as normal, but can under certain circumstances hop back in without the other player knowing where he is. He can tell by something (don't know what yet) though when he fights the unit. The units for hiding maybe even get reinforced. Killing the assassin is the goal.
10. Attack a camp/fortress, or defend it. Can be combined with other of the above options. The attacker may get a bonus in army strength, 30% more point value for example. You can also come up with rules for destroying the palisades/fortress walls, such as hit points for a wall segment (melee and artillery only) or a gate (maybe the attacker has to use a certain unit as a battering ram, such as a Bastiladon).
Both the attacker and the defender have to make decisions. The attacker can use his artillery, or attack the walls en masse. The defender can make a sally to destroy the artillery, or maybe favour ranged units on the walls. Units on the walls can be attacked though, while those behind it can only attacked by artillery.
Maybe there is also a special watchtower near, allowing ranged units to fire without line of sight. That could be a deciding factor in the battle, for either side.
11. For campaigns: Bonus goals. If you dare to divert units from the main goal to get into some corner of the map next to an objective you can get a bonus (such as an artefact/weapon from the GHB or another trait/command ability/other ability/spell. Go crazy, it doesn't have to be balanced that well) The objective could be a holy/haunted place, or a neutral wizard, pretty much the same as in 7. But it grants you a bonus for your next missions as well, which is a bigger incentive to actually do it. It could vanish after a few rounds so maybe you have to be quick.