I've been playing a lot of 'XCOM: Enemy Within' recently, and aside from making me really reeeaaally want a 40K-themed XCOM game, it got me thinking.
One of the mechanics I personally really like is height advantage - that is, if you're higher than your target, you get bonuses to hit. I think this would be a nice little addition to 40K. Say +1 BS if you're one 'level' above the target (i.e.: on a hill, on the first level of a building) +2 BS if you're two 'levels' above, up to a maximum of +3. It would require players to agree how high each 'level' on the battlefield was prior to the game's start, but that's no different to anything else scenery-wise.
What do people think? Other than that a 40K-themed XCOM game would be the greatest game ever made. Which it would be.