What about a "3d" version of space hulk? Or a 3d version, played on a square-grid board, like a larger Blood Bowl board? I think that would work rather well, though it might be tricky to get all the buildings to line up on a grid.
Board games have a nice way of making things nice and clean, rules wise. The inclusion of "unfixed" points of reference makes the requirement for LOS and such more critical, and thus subjective. Determining cover is much cleaner, when you can say, "He's in a square adjacent to the corner, so he'll get a cover save if you shoot from anything behind "this line" and he won't if you shoot from anything on "this side of the line".
Board games, like Blood Bowl, have much tighter control of the movement potential of the pieces, and thus it's easier to create rules regarding movement, such as the "tackle zone" rules, involving blocks, dodge rolls, throwing, catching, assists, fowls... most of those have modifiers for being within 1 square of an opponent. It would be an ugly mess if each rule was "within 1 inch" of an opponent.
I don't think Blood Bowl has ever required an FAQ to more clearly define any rule, that I'm aware of.
I didn't look into Execution force. I have no interest in the models, and Space Hulk has that "cult following" that makes me suspect that it's a better game than EF. So I don't know how EF plays.