When Nokia does return to the market, I see no reason why they could not continue to produce Windows devices. After all, Nokia commanded the WP market, if they wanna try their hand at Android, fine, but why not go for both markets?
I feel that some people are too quick to assume that Nokia + Android = success. Maybe outside of North America, but here (Canada), there is such an uneducated consumer market, as far as the majority of people are concerned, all that exist are iPhone & Samsung Galaxy (and the last of the old Blackberry users who haven't switched yet). Every time I see a friend on Facebook looking to purchase a new phone, it's "Should I get iPhone or Galaxy?". No one here recognizes Nokia outside of memories of the 3310, no one buys Sony's excellent Android devices. A few who research what is on the market, might grab an LG G-series phone or an HTC one.
Nokia's return to market in North America, no matter what the OS is, would require a massive and excellent marketing campaign in order to even have a chance at cracking into the mindshare of Apple and Samsung (the only two choices as far as the average consumer believes). Beyond that, support from carriers to actually offer the device, and not simply have their sales people steer everyone to iPhone and Galaxy S devices (the unfortunate battle that many of us WP customers have experienced all too well).
In terms of production. Does Nokia re-open one of the old manufacturing facilities in Finland? Do they use the plant in India that didn't go to Microsoft and is being shut down at the moment? Do they use Foxxcon? Most likely, they could probably afford to offer two devices - low end and high end.
If and when Nokia returns, they should not return with a conventional device that will get lost in the crowd, but wait until they can bring to market a true next generation device, making use of their crazy R&D, with graphene, bendable / foldable devices, etc. Something that no one else has up their R&D sleeve. If not, Nokia should simply continue through developing future mobile technologies and profiting from licensing agreements to the other manufacturers for use of Nokia's technology.