pbjosh
missing the neotropics
If you are using eBird, you can use the targets tool to see which species different regions hold for you, and how relatively abundant they are... this will answer a lot of your questions. I think it might be hard to get 125 lifers in Middle America in a week based upon what you've already done, but eBird's target tool will be a good place to start.
Aside from pure numbers, San Blas area, Oaxaca, the Costa Rican highlands and 1-2 nearby sites (ie, Arenal and/or Quebrada Gonzalez for instance) would be some of the most pleasant and enjoyable trips you could arrange.
Aside from pure numbers, San Blas area, Oaxaca, the Costa Rican highlands and 1-2 nearby sites (ie, Arenal and/or Quebrada Gonzalez for instance) would be some of the most pleasant and enjoyable trips you could arrange.