You can buy water repellent helmet covers for wet or cold weather, if that helps. So you buy the most ventilated helmet you can find for hot weather and then just put the cover on for fall and winter.
One thing I'd love to see on helmets is a detachable clear face shield like you see on some specialty time trial helmets. Add a tiny wire and battery for defrosting and you'd have the perfect "windshield" for winter riding. No more having to claw your glasses off at an intersection because they suddenly frosted up when you had to stop!