Free placing wouldn't work out (well) for a console RTS as a controller can only do so much, whereas PC you can have a lot of 'controls' to use.
Don't think that is the main problem. I play starcraft with a controller.
There are several problems:
1.Map Design, hw2 maps are obviously focused on leading armies through sometimes narrow corridors or slightly open areas.
2.Gameplay, it seems to me hw2 is obviously more focused on how you lead your army, how you move it through the battlefield. A new free building placement could just be ridiculous different from the way we play the game. Reason I think the game could just be completely different.
Lastly I just wanted to point out my personal view. I think free placement building is an idea that can be developed and explore through a new character that can bring a new gameplay to the table. Not only a character, but a faction. The flood faction could be a free place building idea... But! Only can be place within a radious from the main base or mini bases... Which means you could always be able to "know" where the spread of buildings can be, so you will not be hours looking into the map for a single building lefting to be destroy in order to win.