Actually, Elon Musk teased that during a conference when he unveiled the BFR for the first time with this video. It wouldn't even be that hard. We's have to nuke the planet's poles, and the atmosphere would be much denser and basically terraform the planet. Problem is that the magnetic core is weak, and it would be blown away by solar storms, and be lost back in about a century. We could see it, but it's not worth it. Other methods of doing it are probably too intensive to be worth it now, so you're unfortunately probably right on the last part. But who knows? Science is evolving faster and faster, till then, we'll have to make due with stuff like this and this mad genius at work :)