I would be happy if I could mod Dark Crusade enough to make it more challenging and the battles more fun. Instead of having command and conquer economy you would get there with limited resources and try to make the most of it.
If you play His Most Glorious Imperial Guard then you land on Victory Bay with a fixed amount of guardsmen, vehicles and supplies divided up in regiments and whatnot. Before you attack any region you must choose which companies, regiments and assets in general you will commit to the assault and then you go to battle with just that. You could call in for reinforcements during battle if you have troops on neighbor regions and transports to bring them, also possibly those basilisks could provide fire support from nearby regions too.
As the campaign goes on you could request more men, vehicles and supply for them and depending on your performance and random events you could get them or not or even found them among the planet (capturing and defending those factories now really is important as is getting the support of locals to draft into the lines).
On the field you could deploy some quick structures but that's it. True defensive buildings and HQ's and such would be built once you conquered the region. In addition, during battles your men would have limited ammo (the enemy as well) and vehicles limited ammo and fuel too.
After battle you could get back some of the loses as repaired vehicles and men getting patch back together.
Squads, battalions, regiments, divisions, companies or whatever that survive get experience and small bonus out of it. Lossing men and replacing them with fresh meat will mean reducing the overall experience of the unit.