Hokkaido in Winter with a car : itinerary suggestions for nature and lakes

I really wish I could be more helpful, but this is the best I can do. These websites give up-to-date information on road conditions on Hokkaido's (tolled) highways:

https://ihighway.jp/pcsite/map/?area=area01

http://www.jartic.or.jp/

While this one covers the regular roads:

http://info-road.hdb.hkd.mlit.go.jp/RoadInfo/index.htm

Having never been to Hokkaido, I don't know how accurate/useful that last one is for telling you about winter driving conditions. Highways should be kept in good condition as a priority, and those two websites are - in my experience elsewhere in Japan - kept very up-to-date, and accurate. They'll do you about things like rules requiring chains, and reduced speed limits due to driving conditions, for the highways. Japanese highways are typically useful for getting from A to B, but not much fun to drive on.

I am really hoping to drive around Hokkaido next summer, and it seems like we share some interests, so if you find anything spectacular up there, I'd love to hear about it!

Otherwise, bonne chance. And hopefully some of the other posters on here can help you properly.

Daisetsuzan:
What do you plan to see and do there? I've been there in summer only.

Akan:
A lake is a lake in winter.

Shiretoko:
Narrow roads could potentially be a problem with ice and snow. In fact, ANY narrow roads will here. Despite what Giora wrote (Highways should be kept in good condition as a priority), there is always the factor of the driving ability of people. Hokkaido is not known for the best drivers.

are there any other areas, maybe closer to Sapporo, that can be compared to the above-mentioned natural areas
You want to see mountains, lakes and raw nature in the winter in a land that gets coated in snow. Like I wrote before, I have lived here for 20 years and I come from a snowy homeland in the US. Blankets of snow don't really help provide tourists with wonderful views IMO. It looks the same almost everywhere you go. Some would say go through Furano and Biei, and you can see online pics of what rolling hills look like covered in snow. Hills are hills.

"Daisetsuzan: What do you plan to see and do there? I've been there in summer only."
haha, mountains and lakes :)

"Akan: A lake is a lake in winter."
a lake in winter doesn't look the same like in summer and all lakes are not the same (lake Toya seems horribly touristy with pathways for example).

"Blankets of snow don't really help provide tourists with wonderful views IMO. It looks the same almost everywhere you go."
if the sky is blue, I think the snow makes nature look gorgeous but I understand what you say and lakes and mountains look about the same everywhere in the world (especially if the climate is the same). but, and I admit it's mostly in your head only, I didn't think British Columbia looked like the French Alps at all, probably because I was in a different country.
still, a friend of mine was disappointed in Kyrgyzstan and said the mountains looked liked the Swiss Alps where he is coming from. perfectly understandable and not totally untrue.

the main idea of this trip is to see Japan under the snow and that includes cities AND more remote areas. and I have a point : Japanese people don't look Caucasian :)

If what you want to do in Daisetsuzan is see mountains and lakes, I'd say go to this Facebook site and ask what they think about how to go about doing that.

a lake in winter doesn't look the same like in summer and all lakes are not the same (lake Toya seems horribly touristy with pathways for example)
Find out whether the pathways are even accessible in winter. I honestly don't know. Again, that FB group might help.

Lakes themselves do look the same. It's the surroundings that are different.
Here's Akanko.
Here's Shikaribetsu-ko.
Here's Shikotsu-ko.
Here's Kussharo-ko.
Here's Abashiri-ko.

Whether you see major differences in those pics, it depends on your preferences. What I will suggest is that you check your calendar and see what events occur at those times. There are various ice festivals that celebrate things on the lakes.

As for other areas (like Shiretoko), I would suggest going to Google Images if nothing else, and do a search for location + winter as keywords, and be discerning. Whatever strikes your fancy may be a good place to visit, but you'll also have to determine whether there's a certain time of winter that's best or possible. There are a couple of good websites on tourism in Hokkaido that will offer seasonal pics and events. Seek them out.

some of these pictures look amazing to me and that's exactly what I want to see, water and snow is the perfect combo !

Did you mention that before?

I merely pulled those off Google Images. I have no idea when they were taken. Asking for water and ice is adding another layer of difficulty in helping you. Some people report that Toya and Shikotsu lakes don’t freeze. Google more.

what I should do is think about how to get right to the lakes. as you mentioned, some pathways might be inaccessible. and the more accessible areas are also the most touristy. cruel dilemma.
frozen unfrozen, a mix of both would be ideal.

Hokkaido is a magical, fantastical wonderland in winter: some of the deepest, finest champagne powder snow anywhere in the world (my old host own, when I lived on Hokkaido, averaged almost 9 metres of the stuff each season!).

I would especially recommend Daisetsuzan's Sounkyo Gorge in winter, when the waterfalls have frozen (popular for ice climbing). Also Niseko is a great area to explore by snowshoe. In fact, snowshoes (or cross-country skis, I imagine) can really open up whole areas for exploration, including perhaps the lakes. Haven't been to Shiretoko in winter, but I would expect the same conditions apply (tons of great snow; accessible if you're properly equipped and knowledgable).