I don't want to encourage tragedies. During january, under the week and somewhere remote, a broken leg and no mobile connection will be one. Also, if temperature drops below the alloted one for your sleeping bag, it's game over. If you don't know what you are in for, stay on the WHW. If you have never hiked in Scotland's winters before, also have a plan B in mind which involves heated accomodation. And when planning your mileage, take the daylight hours into account.
Check out the formidable long distance walk page here on walkhighlands for more adventurous ideas
http://www.walkhighlands.co.uk/long-distance-routes.shtmlI don't know your level of experience in the highlands, but you probably can deduce from the walk descriptions and reports whether they are within your limits. Fording rivers has to be avoided at all costs as your feet won't be able to get dry again. Personally I'd investigate Afric-Kintayl-way, Knoydart or Skye without Cuillins for a winter solo expedition that doesn't involve mountains. Solo here also means not meeting others. Walking around Arran might be an interesting winter project also. But that's me. I'm confident with navigation, I like remoteness, I don't need a path to be happy and I wouldn't sleep in a bivy bag. Not for 10 beavers. Whatever you do, stay on the safe side!