That's a seriously good looking wagon.
Predictably, whenever the topic of a wagon comes up, forumites start clamoring for a manual diesel version. As if wagons aren't a hard enough sell in NA!
As for a flat roofline and vertical gate - no question that would maximize cargo capacity. It would also make the vehicle look more like a shoebox with wheels. I'd certainly be willing to give up a little bit of max-capacity cargo space (I suspect I'd rarely load this thing right to the roof anyway) for a BIG payback in swoopy style. Call me shallow.

Hey, if this thing doesn't meet your hauling needs, there's a whole world of boxy and practical minivans available to you. Very flat rooflines and very vertical hatches are yours for the asking.
Jaeger