Before we can fully consider the requirements and trades that go into GMAT's API decision, we need to consider what we want the API to be able to do from an end-user perspective. My best guess of how to proceed with this is to brainstorm a set of use cases, then consider and prioritize them.
This is my brainstormed list of use cases we might want to support (without any consideration of practicality or implementation for now!).
Case 1
Bob is a navigation engineer and wants to use GMAT as the dynamics engine behind his ODTBX studies so he can minimize the amount of modeling he needs to implement himself. He collaborates with Windows, Mac, and Linux users alike, and doesn't want his approach to be limited to a single platform. [this is the IRAD use case]
Case 2
Julie is a mission design engineer doing rapid design studies. She uses an Excel spreadsheet for her design trades and wants to call GMAT behind the scenes to calculate orbit lifetimes.
Case 3
John is an astrodynamics researcher who works in MATLAB and would like to use GMAT's built-in coordinate system conversion tools.
Case 4
Mary is a professor who is developing a custom application in Java that allows a student to visualize simple orbit parameters. She wants to build her simplified GUI on GMAT's base so she can focus on her application instead of validating numerics.
Case 5
Tom is writing a slick new website that allows the public to visualize orbital debris. He wants to embed GMAT's base code in his widget so he doesn't have to reimplement the numerics.
Case 6
Catherine is a mission operations specialist and would like to use GMAT for operational OD inside a facility similar to the FDF. She currently uses GTDS to do...[Russell, a little help expanding this one?]
Case 7
[Nav people, give a use case or two that involves GMAT in a Navigation Integrated Tool Suite?]
Case 8
Hank is a mission design engineer and wants to optimize maneuvers within GMAT while considering navigation results supplied by ODTBX. This will help him quantify navigation uncertainty on the mission's Delta-V budget.
Post more below and I'll combine them all into a master list that we can dig through next week.