I agree with Ich that Mars is the only place to consider trying to live, and that we’d have to do it with external life support. Various SF writers have proposed that we could supplement the Mars atmospher by crashing ice ball asteroids from the asteroid belt into the surface of Mars. Kim Stanley Robinson’s series of novels about terraforming Mars suggested that this could happen within a lifetime. I doubt it.
Now, as to genetic modifications—wow. Let’s see. We’d have to be able to live in a low oxygen environment that is significantly colder and windier (if we were preparing for Mars). Hmm. We’d probably have to have huge lungs (which might involve expanding the rib cage—maybe doubling our chests in size. We’d also need to be able to take in more air, more quickly—widen the size of the mouth? We’d have to amp up the metabolism to generate more heat, and probably make the skin much tougher and less sensitive.
Alternatively, we could ramp down the metabolism, make us smaller and more compact—kind of rocklike, so we didn’t need much energy or food to keep us warm, and it would maybe reduce the air requirements. We could have a kind of built-in windsock that would channel air through us.
Assuming we ever get the technology to make DNA build whatever we want. Personally, I don’t think that’s going to happen. There are too many genes and epigenes, with too many possible interactions. It was developed by evolution, and I really doubt that, except for a few very specific conditions, we will be able to tinker significantly with our bodies. In any case, that takes generations, and human experimentation is illegal in most countries. I doubt if there will be an underground economy where underground labs in the wilds of China buy protection, and try to develop new human forms, but give them no rights, and probably kill them off, to avoid detection. I feel pretty safe in saying this will not happen. Not in 100 years. Not in 1000 years, and not in 10,000 years.
So, to conclude, terraforming will happen first, but I doubt if that will happen either. It’ll take too many resources. We will build in L5 orbital spots first. But mainly for industrial and scientific purposes. If terraforming happens at all, it will have a marginal impact. Or should I say, Marsginal impact? Again, Mars is too far away and hard to get to.
Science fiction is a wonderful thing. And this is a wonderful question for speculation. I wish I didn’t have to be such a wet dishrag about it.