Yes, Outerra is primarily a world rendering engine; support for various types of vehicles is just a matter of integrating the appropriate physics. Since we can't be experts in all areas we want to create a system for plugging-in handlers of physics and simulation subsystems, so that for example the gliding sim community can make their own component that deals solely with the aspects of a gliding simulator, without dealing with world rendering again and again.
FS9/FSX is a good example of how the community and 3rd party commercial developers can extend the base platform/engine.
Could you also consider extensibility with regard to textures, land classes, "autogen" scenery objects models (ie trees, buildings). I guess what im getting at is that your engine needs to allow different regions to be catered for, ie buildings/trees in the South Pacific are going to be quite different to buildings/trees in Europe.
Also, is it possible that 3rd parties will be able to extend the weather system. In FS9/FSX there are addons (eg REX, ActiveSkyX etc) that generate cloud fronts based on METAR reports that they download off the internet. This realtime weather modelling is complicated and best left to 3rd parties but you need to give them the ability to create/delete clouds, haze, wind, rain, snow conditions via a SDK. These weather "cells" might be as small as 1km2 (thinking of high detailed weather around airports, or maybe smoke from bushfires or smog from industrial areas).
Also, what is the max visibility of your engine. I think FSX does 120 NM which is needed for high level flight.
Anyway, you have an amazing engine in the making...
SDK + 3rd party devs = great success for your engine.