I honestly believe that map design could be a lot more open in general if it wasn't for the sniper class. It's not just long sightlines that 95% of the time won't matter for cookie cutter fights, but it's also the width of doors etc. (Which has already been mentioned).
I would also like to point out this is why there's barely ever been any playable koth maps. They're either too cluttered and wierd, or they have gamebreaking sniper sightlines.
A/D Would just have all the same issues 6s have in terms of stalemating & sniper sightlines (Assuming the map design would be similar), except the defenders have no incentive to ever switch off their offclasses to push forward.
If maps were more open, there would be less reason to have a 10 minute shotclock, and it could be significantly lower (because pushing would be easier).
Sniper needs a nerf if we want fewer stalemates and more open chokes & flanks. I would suggest damage falloff + less charging.