My only concern is performance for the large world map. As it is they had to cut down on the full scope for WH2. Not only is the overall performance of that game pretty lackluster, I can't imagine it getting better for WH3. Really hope they finally figure out optimization and can not cut content out of the game because they can't handle the load. It's a pretty bad excuse they've used a few times now. I want each faction to have a large presence on the world map and not be cut down for the sake of technical limitations.
With that said I do agree that's my most looked forward to feature. If they can wow everybody with it this game will sell like crazy. It would be the first time all factions and all these units could exist in one game. Warhammer fans will go crazy if they do it right.