Well, I just had some concept, I didn't actually make anything. Anyway, I thought about something like the original tabletop game where you have to choose your army and then do your mission. I had in mind a cover system and a hiding system (I already made both of them in other maps and they work ^^) plus a RA3 co-commander-like system (for example you could call an artillery strike from your co-commander's Basilisks).
I also wanted to make a system for huge battles in which you had to choose a very big army and then choose something to start with: with the passing of time and by capturing strategic locations you would receive reinforcements and such.
As, for the close-to-codex thing, I tried making EVERYTHING equal to the codex: guardsmen squad are organized in platoons, 1" (should be inch) is equal to 50 range-points in the WE (so a guardsman has a range of 1200 due to the 24"); I changed something like the Leman Russ' range (shorter), otherwise it would have been way too much.
Well, that's it! Like I said, it's mostly in concept stage, but I'll gladly restart making it if you think it was good enough =)