Here is an expansion for Imperial Guard. Of course, I would like to see more than one faction per expansion and I think Chaos vs Imperials would be best for an expansion but as Chaos were so well covered in the opening post, I thought I'd cover just the Imperial Guard (even more so because they are my favourite army). I especially liked the idea of a customisable army where different choices allow different things but you can never have everything.
Here is the tech tree I think would be good (excuse the simple image)
The number in the brackets shows how many "unlock points" (a term I have created) are required to well, unlock said unit. Boxes in Red are upgrades. Boxes in Black are automatically unlocked. The orangey-yellow lines represent the three tiers so commonly found in Real Time Strategy games. Lines leading from a unit to another unit denote prerequisites. A player will never be able to get enough unlock points to get everything. I think about 30ish unlock points would work for my tech-tree.
The Guardsmen need not be the commonly seen Cadians. I thought that the army painter for Imperial Guard in Dawn of War was boring as it was only repaints of Cadian Regiments. Not that I don't like Cadians, but its sometimes nice to see other regiments (perhaps a choice between Steel Legion, Vostroyans, Catachans etc. The only thing that need have different would models be Guard squads and officer squads, vehicles would just different colours.
Command Squad:
The Commanding Officer of an Imperial Guard army is required to keep the troops in line. This is what the officer should have looked like in Dawn of War 1, but for some reason they turned him into some sort of imitation Space Marine commander. Imperial Officers don't get involved in fighting, that is for the cannon fodder lesser troopers.
Heavy Weapons Teams:
These act as heavy fire support to take out targets that regular guardsmen cannot or to soften-up enemy squads before the lasguns start firing. Mortars in particular will be potent in the hands of a skilled commander as they can hit targets they cannot see, forcing the enemy to retreat or advance, particularly useful for a defensive army such as the Guard.
Snipers:
Adept a picking of heavy infantry that lasguns would struggle with. Can infiltrate to go invisible.
Hellhounds:
Armed with a turret mounted Inferno Cannon, Hellhounds can toast infantry with ease and makes their cover useless. Why this wasn't in Dawn of War is beyond me. Its a flame-tank! Who wouldn't like them?
Demolishers:
A variant of the Leman Russ that has short range but a very powerful battle cannon and more powerful sponsons including multi-meltas and plasma-cannons.
Leman Russ Vanquisher:
The Vanquisher has a very powerful long range anti-tank battle cannon that can tear through even a titans armour with little effort. It is a very rare variant though.
Gorgon Transport:
Guardsmen are lightly armoured and can get shot down easily at long range before they can even be effective in battle. Chimeras can only hold 10 men. The Guard's solution is the Gorgon capable of holding 50. The Gorgon is useful when the Guard go on the offensive. And yes, those are flamers you can see on the sides.
Phew, I may add more later, but if the Guard were put into a Dawn of War 2 expansion I would want it in this sort of fashion and to be awe-inspiring, not looked down upon.