Well if any unit is possible, how about a Dragon riding Champion. Or a unit of fire throwing gecko capable bletching fire 20 feet. Or a elephant with a cannon on its head. A flying Disk for your mage to ride
.
Swarms of rats, to weaken up and destroy lesser foes. Munch minions munch till those elves P*** off.
A Troll phalanax, yeah they maybe big and dumb
but if you tell um to hold a spear and stand together surely that would work.
The Chosen -Frenzed, heavily armered swordmaster, you got no control over them, they just attack anything close to them. Of course they are hard to hurt and kill almost anything they attack but your enemy could lure them away from the main battle and hammer them with missle weapons.
Warmachines, I know their some people here who don't like fantasy mixing with gunpowder but i'm not in that camp. Cannons, Trebuches, Bolt throwers, A goblin thrower
, troll thrower
, a cat-apult
.
Hordes of wolves lead by a 'packmaster' wolves attack and panic the enemy but only as long as the packmaster can control(until he dies) them after which they go crazy and attack everything.
Armerd bears armed with Giant Hammers.
An undermining machine, think Giant drill powered by steam. made of hard nasty brass, used to drill under cities walls.
Lizard men with scales as hard a steel, blades sharp and strenght unmatched, but who are not skillied at swordfighting (i don't need to parry when i can smash your sword in one swing).
Super Halfling Slingers wiith admantium weapons, Got go with the classic.
Pox throws, evil men in capes who throw bottles of bacterial culutres at the enemy, no damage done but enemy unit is weakened.
A molten iron chariot, a chariot carring a big cauldron of moltern iron, it charges and spills it all over itself and the unit. Chariot dies and unit gets melted
.
"so you could have, something that resembles those uber worms in Mom" , awesome Idea the only thing greater than a Great wyrm would be a hero riding Great Wyrm or having a cannon on its head, or perhaps two.

I think i'm getting a little carried away with myself. Of course this is what I'd really like, and though i don't want to jynx, it i have a feeling most of what I put above would not be capable in your generic unit designer, but it is fantasy. Anywho Can't wait for the June beta(?).