That's awesome =D
The chain reaction effect, though, would allow room to make the tanks more powerful in the end, but it's all a matter of opinion on that part.
True that the Burning Oil would cause a shitload of lag on that part O_O. Hey, have you ever played Command and Conquer Generals? If so, then you'll know exactly what I'm talking about. If not, look up China's Dragon Tank. Basically it has the ability to create a Fire Wall which spreads about in front of the tank. Something like that would be cool, but difficult to create (could be based off of Stampede, but made so that the summoned 'stampeders' all start from one point on the caster and come out in a cone). Just an idea, anyway, I have no idea what other races you plan to include, tho
BTW, I added to my post with a not-so-brief note on the Great Sword and its abilities. Just mentioning that when massed, they are the most op melee unit EVER made in the game :O
Good luck with the project, btw! =D