Blender can import FBX and obj so for vegetation you could use any of the popular software like speedtree or Xfrog. There is also some scripts within Blender for simple tree and shrub generation. Don’t expect anything comparable to Speedtree though. There are no parametric objects like roads and bridges and similar, but Sverchok plugin could be used (maybe) for that.
That said, I just finished a landscape and architecture project of medium scale completely in Blender, and frankly wouldn’t do it again. I lost too much time with little annoyances that build up on a bigger project like no real grid snapping, no parametric objects built in. Modifiers depend on object scale (whats up with that anyway) so you can’t for example have a object non-uniformly scaled that shares mesh data and apply bevel modifier because the modifier distorts the bevel etc. Also there is limited ways to modify an instance. You can’t have different materials on instances if the materials are on mesh data level.