There was quite a bit of talk (from him) about moving back to the 400m. I think at the present time he'd need to lose some weight to do so and be very successful.
Personally, I think his best bet is probably staying in the 100m (where he could be as high as 2nd in the world) or moving to the 400m (where he could maybe get on the medal stand). I think the 200m is too deep right now for anyone other than Bolt, X, Gay, Spearmon, Merritt, and maybe Crawford to be thinking about making an impact. To get on the medal stand in the 200m looks like it will take about a 19.75 (on that day…not necessarily a PR). There's already 3 guys who can definitely do that. In the 100m, an athlete could probably medal with a 9.90 which Obikwelu could easily run.