It's hard to say what will happen to him. But nullius pointed out something very important: " By what right does the United States presume jurisdiction over Assange? He is a not an American citizen, and is thus not generally subject to the personal jurisdiction of American laws. He is not alleged to have committed any acts within American territorial jurisdiction."
I really wonder how this will be explained during the trial.
I'm no expert, but my naive understanding is: Assange is accused of hacking into US databases and publishing sensitive security information, thereby 'endangering lives', i.e. a crime against the US and US citizens, so answerable in the US. The UK has an extradition treaty with the US, and for the hearing to go ahead, the judge in the UK just needs to be convinced that the alleged crime is one that might have led to trial here, had it occurred in the UK. There is
no consideration of guilt here, merely an assessment of the nature of the
alleged crime. The US just needs to convince the UK that there is a case that needs answering. I believe there is something in there whereby the UK won't allow extradition where the accused could face the death penalty, but I don't think the US want the death penalty anyway in this instance...
It may be a flimsy pretext, but in practice that doesn't matter. The UK will perform as instructed.