I doubt US will do any major expansion to the caribbean from MIA, it has been dominated by AA for years and US would face major competition from AA. However, if you think US will ultimately expand in MIA, i would like to see where.
As far as Star, MIA is the major gateway to Latin America and South America and with the new terminal will strengthen that gateway. However, in MIA no Star carrier offers a great deal of Caribbean destinations. So now with US entering Star and easily having the largest pressence in the Caribbean wouldn't this make PHL and CLT the major caribbean gateways for Star?