This scenario is very unlikely.
What would the U.S gain by the annexation of Nicaragua, it would have to pump in alot of money to keep that very poor nation afloat.
As, said before, the Panama canal is already built. Perhaps, but this is a very long shot, maybe some powerful elements in the U.S feels the need for a second canal in central America. For "Insurance" against the first, what happens if the the Panama Canal gets bombed, natural disaster, etc. So that shipping is not entirely interupted.
But that is rather implausible.
Trying to annex Nicaragua would be very difficult, and would probably instigate further guerilla actions/instablity in Latin America. Hell, maybe the Panamanians get irritated also.