There is not a shred of evidence to show that the US had any intention of kidnapping Assange. We are in the realm of pure conspiratorial speculation here. ("They are bad guys; this would be a bad thing; therefore they would do that thing.")
Can you give any examples of this happening? Particularly given that we have access to the same diplomatic cables from the US embassy in the UK as the Swedish one which showed continued rendition flights from Sweden?
Zooming out a bit, there is very little analogy between the Agiza/Alzery case and anything that could conceivably have happened to Assange. Ahmed Agiza and Muhammad Alzery were asylum seekers whose asylum applications failed and who were then deported to their home country. The correct legal procedures around deportation were not followed (and of course the Egyptians broke their promise not to torture the two men), but it is a huge stretch to try to make an analogy between this case and some supposed plot to kidnap Assange once he was in Sweden. The latter would have been a vastly more flagrant breach of the law than a rushed deportation, and would have made a mockery of Sweden's original extradition request.
The simple fact is that it is extremely unlikely that the US would have kidnapped Assange from any location.