Hmm, I don't have an idea, but I get put on the spot to DM all the time and do ok, so let's see if I can give you any help on a plot.
Hmm, setting first. . .
Alright, for factions, the Veiled Alliance: Elves, Humans, Dwarves, and Orcs (this is assuming that a faction can have more than one unit type, which I'm thinking, from how the units advance, should be possible). The Sahdaar (just a random name, a drunken monkey can probably come up with a better one) Skeletons, Demons, Goblins, and Shdaar cultists (either humans, or a mix of the opposing faction's models in like black and red robes)
Story: A shadow swept accross. . .gah, writer's block on a cool fantasy continent name. . .and nearly had all the world in it's grasp before a last, desperate alliance between all who called (insert cool continent name here) home, former foes bonded by a fell assailant greater than any of them. Even as hope began to glimmer upon the horizon, fate once again stretched out its cruel hand and snatched it away, for the alliance learned that they had been betrayed, a small faction known as the "Cult of Sahdaar" had secretly begun to worship and serve the invading evil, seeking one who could be made into a vessel for the master of this dark force sweeping across the land, that he might himself step onto this world and bring his full power to bear upon its subjects. In a desperate battle, the Alliance spent the better part of their strength to pierce the dark horde to their unholy temple interrupting the ceremony near it's completion. The subject, previously a simple peasant, had already begun a hideous transformation. However, the interruption broke the focus of the dark priests, and only a portion of the dark being's power had filled the vessel, the rest was expelled back into the rift from whence it came, shattering the altar at the top of the temple and killing all who were there. . .all but one. The dark power protected him, and he was not slain, but plummeted from the high altar, and was lost to the waters of the great river Thimuvais, or Dal-Thurin as the dwarves call it. Nearly a year passed with the two forces locked in strife, much of their strength lost with the death of all those at the dark temple, and neither could gain advantage over the other. One day, there was a battle between the two sides, nay, merely a skirmish, and would have easily been beneath mention, had it not been for one thing. A small group of dwarves valiantly fought a fierce pair of demon lords, and in the end, the demons fell, their dark hearts cleft by dwarven steel, and scattered about lay the dwarves, dead nearly to a man, all but one. This one lay, his body broken, breathing his last, by the sea, longing for sight of the mountains before he passed he instead beheld a terrible sight. From the stones covering the shore, a figure rose, and the dwarf beheld the twisted form of the dark vessel, a vision of terror that sent a rush of urgency through him, and the dwarf found the will to live, slinking away to carry word to the alliance that the Vessel of Sahdaar had survived, or another was being made. The vessel himself/herself staggered up the shore, casting about, a short sword was found unbroken, and a handful of coins. The vessel woke, not knowing what had happened since the battle, but remembering all that had happened up until that point. All was a fog, even his/her memories of their simple life before, another voice spoke in hissing tones, and the strength that flowed through his/her veins had a heady aroma of power to it. The vessel soon realised he/she had been forged into an immortal body, to contain whatever soul of malice led the dark forces. Instead, he/she had been ripped from the dark power's grasp at the last moment, his/her body in new and terrible form, but his/her mind and soul still his/her own. This left him/her on a threshold, for he/she held the power to turn the tide of the war. With him/her before them, the forces of the alliance could drive back the hellish shadow armies and reclaim their world. . .but the touch of dark power had left an alluring vision, that the world could be his/hers, and the twist of fate had given him/her the power to marshal the dark armies, and finish the conquest of this world. . .his/her world.
Main Character Idea: Looks like one of the alliance races, but deformed, with scales, maybe horns and bone-spurs. May need to simply go with human (or any other single race) for simplicity in design's sake. Anyway, the storyline gives a good set-up for the player to side with either faciton, and a reason why he tends to just "wake up" after a lost battle.
RACE/UNIT IDEAS:
Alliance:
Humans - could build much like the basic swadians or vaegirs in the base game, becoming powerful, heavily armored mounted knights or sharp-shooting crossbowmen with excellent range and accuracy.
Elves - light and fast, but not very tough, based mostly off of the peasant women that become sword sisters. However, they could get bows instead of crossbows, and never wear heavy armor or wield two-handed weapons, instead, keeping to fast weapons and focusing on mobility.
Dwarves - slow and hardy, even new recruited dwarves are in medium or heavy armor, and they have an exceptional number of hitpoints. However, they can never be mounted and move half as fast as normal human units.
Orcs - Barbaric and physically powerful, orcs scoff at other races hiding behind armor. Orcs are large, and tend to have better reach than other races, aside from demons (and Gnolls, if neutral armies are involved), they also recieve a large melee damage bonus. However, they cannot wear armor.
Sahdaar:
Skeletons - Much like humans, but with fewer hit points. However, their attack speed is slightly faster than normal, and they gain an armor bonus against piercing attacks, making them almost completely ineffective, even against completely unarmored skeletons.
Cultists: Random mixing of alliance races, however, they also recieve random armor and weapons (could be good, an orc with armor and a heavy melee weapon, or bad, a dwarf with cloth armor and a spear. . .)
Goblins: Smaller than normal, so hard to hit, and move quickly. However, they cannot use two-handed weapons or mounts.
Demons: Large and extremely powerful, they cannot wear armor or ride mounts, but their attacks deal status effects, low level deal damage over time, and demon lords add knockdown to their attacks.
Neutral:
Gnolls - Large creatures that can only wear light armors and can't ride mounts, but move faster than your normal person, and have a long reach.
Kobolds - Small creatures, so hard to hit, they don't move fast like goblins, but they have increased damage with thrown weapons, and have innate armor that adds to any armor worn.
OK, that pretty much sums up the idea. Best I could come up with on the spur of the moment, but it should help get the ball rolling.
