Since it's been a month already, time for an update.
I had the custom map projection working at the beginning, drawing continents, borders, cities, labels, bases, craft markers, radar ranges, missions and their areas.
Now instead I have a mod loader that correctly loads and merges the OG ruleset and all the mods enabled in the options plus loading whatever resources that might be needed for the geo/basescape. So that the whole thing would work with whatever modset one prefers.
The modloader itself might be useful for finding stupid errors and typos while modding, it has some basic consistency checks and is verbose about them. Many more can be added (I don't really know what might be useful).
Next step would be to redo the renderer to a) drop SDL_gfx in favor of GLES/GLSL (SDL_gfx is no good, I could actually see the triangles being textured at each redraw) and b) switch between original (orthographic) and arbitrary cylindrical (miller, mercator, whatever) projections at will. Horizontal ( logntitude-wise) scrolling worked already, it will be grandfathered in. Vertical (latitude-wise, as when one wants to take a look at the pole) scrolling in cylproj mode will be done too.
After that the whole OG mission logic will be reimplemented, to a) learn how it works down to the last detail and b) see what's the best way forward for the new AI API so that it can be turned on and off at will (depending on a mod).
Detection/Intercept UI will be reimplemented at this stage too, starting with OG's one, to see exactly how inadequate it gets given massive increase in flights detected.
It might further be necessary to do the basescape, so that the whole project gets to be a semi-playable half of a game, without the battlescape (tactical mission results decided by RNG plus maybe some function defining abstract strengths of opposing squads, taking into account terrain, races, geoscape-level morale, whatever).
If all that leads somewhere, there would be a fork of Meridian's tree with the new AI support. That's the long range goal.
These are the plans.
I'll be putting up the code on github shortly, I have to whack the SDL_gfx graphics code into shape so that it at least displays something again. Just a question of adapting the draw code to the modloader API from an ad-hoc one.