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I already know that the Freespace-2 engine does not support branching, but does the WCS FRED Freespace-2 editor support ground-based missions in addition to the space and atmosphere based missions found in WCS?
The engine does support branching very well. We just decided against it because a main branch would have been a losing branch and since the eingine doesn't support save games, just a progress save, we didn't want a player to end up beyond a point of no return. but the engine and the campaign editor support any kind of branching just fine.
Regarding ground based missions, attempts at simulating gravity have had mixed results over the years, although I don't think Wing Commander had gravity either, at least up to WC4. As for the planet's ground, the usual solution is to have a big "ship" that acts as the planet's surface and an atmospheric skybox.
We tried that. Results were.... let's just say... don't try it. The engine cannot handle it properly